How effective would it be in combat?
What kind of effects might it have against full metal plating compared to a normal ball and chain?
You won't gain a benefit when hitting soft targets.
The advantage of the dead blow hammer is the distribution of the energy over a longer period of time. This significantly helps prevent rebound when striking a rigid surface.
Essentially the dead blow hammer turns your hammer strike into a really solid shove. This is not what you want when fighting someone in platemail. You want to dent the plate and potentially crease it. The plate is already trying to smooth out the impact from blows and a dead blow hammer or mace would only aid the plate in this. There is a reason dead blow hammers are used in body shops for chassis work, they don't damage the sheet metal.
However, if you wanted to make a sparring weapon that wouldn't damage the plate too much, but would still simulate blows, a dead blow hammer or mace would be an excellent start.
In general, as armour got heavier and more effective, slashing swords went out of fashion as it was too difficult to cut through armour. Polearms with greater leverage, "smashing" weapons like hammers and stabbing swords like rapiers evolved to negate the protective attributes of armour.
The problem with a "dead blow" weapon is the force is distributed in both time and space, and diffused over a wide enough area that you will not be able to deliver a blow that would take out the opponent. The best you could hope for is to knock them down (and maybe follow up with a dagger), or if you are lucky, a blow to the head might stun them long enough to subdue them, or give them a concussion and put them out of the fight. Of course a knight or man at arms is wearing a great helm, balanced on a padded ring (a primitive suspension system, much like modern helmets have padding or straps to keep the helmet proper away from the head), a layer of chain mail (the coif) and possibly a leather skull cap as well, so it is easy to see why war hammers or halberds were favoured. Even a mace was usually made from multiple triangular "blades" around a central shaft with the points out to concentrate the force of the blow.
The best use of such a weapon as a dead blow flail would be if capturing the lower ranked levies is somehow important. A spiked flail such as the one pictured would cause lethal damage to the peasant levy called up into battle (generally unarmoured and trying to fight you with a pitchfork or billhook), so a dead blow flail would knock them flat with maybe broken bones or concussions, allowing you to scoop them up as captives. A team of people would be involved, one armoured person to wade into the mob and start knocking them down with the dead blow flail, while the rest of the team rushed in and grabbed the captives.
I̶ ̶d̶o̶n̶'̶t̶ ̶t̶h̶i̶n̶k̶ ̶i̶t̶ ̶s̶o̶ ̶m̶u̶c̶h̶ ̶m̶a̶t̶t̶e̶r̶s̶ ̶w̶h̶a̶t̶'̶s̶ ̶i̶n̶ ̶t̶h̶e̶ ̶b̶a̶l̶l̶ ̶a̶s̶ ̶h̶o̶w̶ ̶h̶e̶a̶v̶y̶ ̶i̶t̶ ̶i̶s̶.̶ ̶S̶a̶n̶d̶ ̶i̶s̶ ̶l̶i̶g̶h̶t̶e̶r̶ ̶t̶h̶a̶n̶ ̶m̶e̶t̶a̶l̶,̶ ̶b̶u̶t̶ ̶y̶o̶u̶ ̶c̶o̶u̶l̶d̶ ̶c̶o̶m̶p̶e̶n̶s̶a̶t̶e̶ ̶w̶i̶t̶h̶ ̶a̶ ̶l̶a̶r̶g̶e̶r̶ ̶b̶a̶l̶l̶.̶
A proper dead-blow head spreads the force over time and reduces its peak force, which in general would tend to reduce damage effects.
As for not having spikes, there are historical ball & chain weapons like that too. I don't think it's a huge difference in effectiveness, but of course the spiked version will cause shallow puncture wounds if they get through whatever the target is wearing, it looks a little nastier, and it focuses force on points. I'm not sure, but I think in the case where a hit doesn't penetrate armor, the plain ball might have more effect at the same weight, as I think it would more directly concentrate the impact on one point, instead of two or more spikes splitting the energy.
Against enemies with armor you'd imagine it would be fairly ineffective unless you hit them in the head. Even then, the weight it would need to be to stun them significantly so you could follow up with another weapon would make the mace fairly unweildly. I would vote to keep the spikes.
What's the context for this? How heavy is it?
If I had one on top of a wall it would probably be great against people trying to climb up ladders. Against less nimble foes it could also be effective since it would be hard for them to get out of the way.
However against someone/thing that is very agile if you missed you run the risk of being thrown off balance by the weight. The recovery time for something like that is also probably quite slow. Much longer than quick slashes of a short sword or some other more agile weapon. So if if short sword wielding, agile opponent was able to easily dodge your swipe a stabbing is probably in your near future.
As far as damage against armor that depends heavily on the weight of the flaily bit and the composition of the armor. If there is any kind of significant padding between the metal and the body of the wearer the effectiveness could be greatly reduced. If it's just bare metal and fits relatively close to the body it could certainly break bones with enough force given the flaily part was heavy enough.