Honestly, there isn’t really enough information in the question about how they’re kept aloft to really distinctly answer without making assumptions.
So, as such, I’m going to make a few for the first answer. Namely the unobtainum engine doesn’t scale down well enough to work in conventional aircraft (i.e. say, weight limit 100 t or so, I dunno.) .That it doubles as a propulsion source to give your airships comparable sea ship velocities at seaish level. Since it doesn’t scale down well, and can double as a propulsion source, I sort of assume that it produces its lift as a discrete force where it is, as opposed to a lift field or something.
I will probably edit the response if you give a bit more info about precise mechanics/limits/to clean up the answer. Anyway...
If we work under these assumptions (which give you functionally WWII naval ships in the sky), you end with a pretty scary military unit that still has some glaring vulnerabilities.
You're looking at more survivable ships of simmilar size mounting a more spread out array of light guns (4-8 inch, depending on size), and depending on the class, the top deck will likely mount some medium and capital armaments for long range ground shelling and heavy airship brawls. Overall, there's a focus on smaller more rapidly firing armaments that can more reliably score hits on other airships and keep conventional aircraft at bay.
Because long range combat involves elevating your guns upwards anyway, the heavy armament pretty much stays on top of the ship.
Aircraft have a lot more trouble on the offence, since they're functionally lose the altitude advantage they have against surface ships, and near misses no longer cause nearly as much damage, if any at all. It's more than likely that specific anti-airship attacker craft are developed.
Transporting a lot of cargo across treacherous terrain becomes a lot easier, and lack of air support of any variety becomes a huge threat to ground forces, especially in open terrain.
They're good, extremely useful assets, but by no means infallible, and aircraft are by far still a huge threat to them. But a lot depends on how high exactly they can fly.
Now see, in normal ship operations, most of your gear doesn’t work very well underwater, this is why when you sink a ship, it functionally becomes a non-threat. Cannons don’t function well under water at all, and the electronics and engines don’t fare much better, and it doesn’t help that the crew need air to not die. Generally, sinking a ship means poking enough holes in it that enough of the air inside the ship gets replaced with water, the ship becomes negatively buoyant, and it goes into the largely unstoppable process of sinking.
If we have our airships however, the only real way to disable them completely is to nail enough of the structure that it falls apart, or, destroy enough of your unobtainum so it literally drops out of the sky/flips/capsizes. Both of these are more likely to occur at much more dire circumstances then what would sink an ordinary ship, since well, you’ve hopefully armoured your ship enough that doing significant damage to the structure requires a lot of damage across the ship (meaning really, magazine explosions would be the most potent damage occurrences). The other method, damaging the reactors (because really, you’d need more than one) is difficult because they’re most likely the best protected component of the ship, likely mounted as far from the extremities of the ship as reasonably possible.
Of course, just with real ships, disabling it doesn’t always mean completely destroying it. A ship that’s set on fire in the air is likely more vulnerable than a normal seagoing ship, as while you could potentially operate at high altitudes where keeping a fire going is difficult, there’s the fact that such a fire would be higher risk for the crew and significantly more difficult to put out, leading to a higher risk of ammo cook-off.
The end result is that your ships are arguably more durable than a conventional seafaring counterpart, which is saying a lot considering just how much punishment some WWII ships took. However, while this is somewhat a defensive benefit over seagoing ships, there’s a much, much more prominent one at play here, and that is the agility.
See, with naval ships, you functionally only really have two dimensions of movement, and even then, you’re limited to sort of forwards backwards + turning. Even if you assume the minimum increase in movement, i.e. altitude gain cannot be done independent of forward propulsion (which is kind of unlikely), you’re still dealing with another dimension in which they can perform evasive actions.
Functionally, this squares the area in which they can be in any given future reference point, which makes them significantly harder to target than a comparable seafaring opponent. Not only that but high explosive near misses designed to work on surface impact are no longer a thing, meaning even more precision is required to land even glancing hits. Without the water to carry the shockwave, combined with the fact that shrapnel isn’t exactly the most effective weapon for dealing with thick steel plate means that there is a huge jump in durability from this simple adding of a dimension.
It once again depends on how the unobtanium works, but the altitude limits could have a very strong effect on the interplay between these ships and other military assets. But I’ll cover that after I cover their armament distribution.
One of the primary disadvantages about dealing with the third dimension is that now you functionally have to look in a new direction, downwards, especially if you want to use your airships to project force to the ground. However, cannons firing at longer ranges still need to tilt upwards in order to maximise their range, something that would be severely limited by mounting them on the underside of the ship.
Of course, when functioning at long range, mounting your cannons on the top is ideal. It gives you enough elevation to fire at longer ranges, and well, the shell’s arcs are parabolic, so as long as what you’re firing at isn’t too close, you can just shoot from the top deck anyway and have it fall back down to your target.
This fact, combined with the prospect of WWII technology (Heavily armoured moving casemates with full rotation on the bottom of the ship would more than likely not have enough structural integrity to not fall out) means that weaponry mounted on the underside would most likely be smaller dual purpose armaments for precision bombing or dedicated AA weapons designed to suppress enemy aircraft climbing to engage.
Similarly, such armaments would likely be installed on the sides (front, rear, and port/starboard, depending on the dimensions) of the airship as well, to allow for engagement of other airships at closer, lower elevations.
Indeed, even on the top of the deck, it would be more likely that there would be a higher number of smaller calibre guns with higher rate of fire as opposed to the generally larger battery arrangement found on surface battleships, as a product of both the increased weight from the secondary armaments on the side and bottom, and the fact that air engagements at longer ranges would be significantly less practical, due to the higher degree of movement capable.
Hence, there would be less of a precedence towards all big gun battleships, and instead a likely heavier focus on heavy cruisers or somewhat smaller ships.
Despite this, big gun battleships would still have a prominent place, as their increased range of fire would still offer an advantage in engagements at longer ranges, and the increased shell payload would still play a role in surface bombardment and the occasional lucky hit on a ship. This is doubly so, as a battleship with long range artillery guns at high altitude could potentially shell a city or other large target with complete immunity any form of ground based counter attack. Only aircraft or airships would pose a threat at all.
Unlike WW II, a big difference would be in the lack of submarines and their torpedoes (and well really torpedoes in general). Since you’re now in the air and we assume that the unobtainium wouldn’t feasibly work on a small object, not to mention that WWII era torpedoes don’t have seeking guidance (rather, they could turn to face a direction and that’s it), combined with the fact that there are now three dimensions of movement means that even if they did work, they’d be functionally useless. Honestly, I have no idea how they managed to actually score so many hits in WW2 with them from ships (well I sort of do, 2 dimensions and one direction of travel means you can shoot a whole bunch along the path of where the enemy is going, and hope they don’t notice/can’t evade in time). Either way, they’d either not exist, or be worthless for hitting anything.
Most of the other ship classes would probably exist however. Carriers would be obvious, and the entire gun toting-warship line would also exist, considering that you don’t always want the biggest gun, but sometimes you want to bring a bigger gun. Speaking of carriers, the interaction between these an aircraft would most likely end up similar, but significantly less advantageous towards the aircraft, for a few reasons.
See, the problem with the vast majority of WWII aircraft is that most of them can only really deal significant damage to such a warship via the use of bombs or rockets (for smaller ships or more vulnerable subsystems on larger ones). And during WW2, neither were exactly the most accurate of weapons.
And the star of sinking ships in WW2, the torpedo, is pretty much absent for much of the early war, assuming the later inventions (glide/radar guided bombs) are invented later in the war. Indeed, this is probably the biggest factor that lowers the number of airship sinking occurrences until more advanced weapons tech picks up.
With the higher evasiveness of airships, you’re looking extremely low hit rates on bombs, especially now that glancing hits have a lot harder time causing damage. And rockets for a good degree of accuracy require doing the same sort of suicide run that a torpedo bomber needed to perform, namely, flying directly at the target to ensure the airflow over the rockets is stable and their accuracy is at its maximum. And unlike torpedoes, where you only need to match two dimensions of movement, rockets arc on a trajectory, and with the airships, would need to account for it's additional directional movement.
The biggest difference however, is the altitude that the aircraft have to tackle them at. See, a big aspect of aircraft performance is the operational altitude. Many of the attacker aircraft that were used to perform high precision airstrikes worked well primarily only at lower altitudes where their high lift co-efficient made up for the fact that they were carrying potentially more than a ton of ordnance.
This effect becomes significantly more prominent the higher our airships can effectively fly, since the higher they can safely go, the more of the agility they can functionally rob from the attacker aircraft, making them easier to intercept or shoot down, especially given that they would need an altitude advantage over the ship in order to effectively bomb it using freefall ordinance. Once we start getting real high up (about 10 km up), it starts becoming more of only a few aircraft can functionally reach your airships in the first place.
Cloud cover would also play a significant role in attacking airships, as the double blind scenario it causes can effectively prevent any attack on the ship while it is enshrouded, but at best it can only burn the opposing side’s fuel reserves as they wait for the cloud to disperse or the ship to move out of it, and at worst, results in a aircraft crashing into the ship while both are moving through the cloud.
Regardless, despite the sum of effects, aircraft would still function as an effective fighting force against airships, contributing as one of the larger killers still, if not the largest. This would likely be the result of divergent development of weapons technology.
Given their (I assume) prolonged presence in the setting, even at a WW2 tech level, there would be divergences in terms of weapons development, especially in relation to aircraft. Development of rocketry based payloads would likely see more aggressive development, along with smaller more stable glide bombs for hitting airships (functionally replacing the surface based torpedo payload option with either large rockets or glide bombs).
Not only this, but given the significant risk increase in fires on board airships, there would be a higher focus on development of dispersion weapons designed to set alight the airships with a smaller or lighter load than conventional high explosive payloads. The use of clustering or area dispersal weapons in particular would’ve likely seen more significant development, allowing a craft on a quick attack run to still potentially cause some damage to more vulnerable or exposed systems on an airship.
Not only that, but the prominence of cannon mounting aircraft such as the B-25-G, Ju 88p-1, Mosquito FB Mk XVIII and Hs 129B-3 etc. would’ve likely been increased, as the relatively accurate higher calibre cannon fire would still have an effect on the larger more armoured airships, and they would’ve likely seen more advanced development in building more capable airframes to mount such airborne weapons.
In terms of surface based equipment, minimal forest cover could help significantly camouflage stationary weapons batteries of artillery calibre anti-airship guns, though such mountings would most likely be extremely vulnerable to counter assaults, especially from the ground. Functionally, they’d have the advantage of being significantly more spread out and better hidden, but lack the resistance to direct attacks an airship would have. They’d also probably be a lot cheaper to produce than an airship with comparable function. You’d still need to score direct hits though, which would limit their effectiveness, but airship to airship combat has the same caveats (bar the fact the airships can close the distance to increase both risk and success rate).
Of course, once the more advanced guided rocketry and jet engine technology started running around, they would seriously start feeling it, just like their original surface based equivalents. Their lower vulnerability to sinking would help keep them going for longer, but it would give everything a reasonable method of delivering a lot of punch with accuracy, at which point, the added armour isn’t nearly as helpful. Those ground based hidden batteries would only need one shot to potentially cripple any counter offensive, and their large profile makes detection easier and landing hits more so, while they can’t counter the much smaller, hidden opponents first.
As an asset
That being said, at a WW2 tech level, they would make extremely good assets. They would combine a Heavy bomber and warship in one nicely armoured package, and would have minimal limitations on where they could be used. But their transport capabilities would be equal to, if not outshine, their combat capabilities. The ability to land thousands of tons of war material at anywhere large enough, ignoring the intervening terrain, would change the face of wartime logistics.
Depending on their operational altitude, they could safely travel in fleet formations that would be vulnerable only to large aircraft formations or other opposing fleets of airships. Air combat would become a lot more significant and yet, intercepting airships would more a more costly move than intercepting warships. The lack of a “stealthy” attacker like a submarine would mean that commerce raiding could only really be done by opposing airships or, in the case of supply fleets not supported by escort carriers or air assets, opposing aircraft. The high launch altitude would give the defending air forces a significant edge in rapidly responding to enemy attacks.
Despite this though, even with their usage as mobile armoured artillery, conventional ground and air combat would most certainly remain a central aspect of battle. Aircraft would still retain their relatively extreme range compared to artillery weapons, even if airships could travel through the sky. And in the end, most of what wars are fought over is the ground territory, while you can project force, troops, firepower etc. with your airships, you can’t really head in and secure it without ground troops or vehicles, especially if you want to leave any of the infrastructure intact.
Identifying fleet numbers, their locations and whatnot would be a significant part of countering them. After all, each ship can carry potentially thousands of paratroopers alone, and have enough artillery to provide support until more reinforcements arrive. Visually spotting an airship at day might be easy, but once night falls, detecting them becomes much harder and radar detection becomes a more vital aspect.