How do you avoid this from becoming something that they can apply to every situation and completely overpower every challenge or other species that they face?
Give then challenge good enough.
Yes, we are not totally hivemind, and it's not so easy to imagine challenge or situation, without thinking about details of a potencial situation.
But for our future and our future generations it's better and more fruitful not to nerf hivemind, but investigate their strength and weakness.
They could be our future, or they could be our enemies - in both cases, it's good to know weaknesses to avoid or to exploit.
Hivemind isn't god. If you see them as a perfect creature, it just means that your perception of how they work is wrong. Some are excited about it as some sort of personal immortality - they are wrong too. It's the same kind of immortality as to say you will live in your children, yes to some extent that's true. Or it is the same as to say, humanity is immortal, yes, at some extent per aspera ad astra.
Investigate, and create them through challenges. It's not enough just to say they will do that easy because they are hivemind, but how they do that, how exactly they will do that. We can't think the same way and at the same speed as computers do, but we investigate code, we test code, we create code through testing, debug it etc - it takes time, huge time compared to execution but still we can understand that code.
Information exchange speed limitation - 1c, is a pretty reasonable limitation. Less not needed, faster there are other challenges in terms of understanding how they may work (more pure algorithms stuff).
You will find inspiration here Swarm intelligence - most examples do exactly what hive mind have to do, processing information bigger than they can fit in their individual brains, with a low(limited) bandwidth of data channel, and low(limited) speed of data transfer and processing.
Also, databases algorithms about optimizing data storage and reduction of time access to data can be inspiring too.
P2P decisions making and data storage's, as an example well-known bitcoins - they are working model of such system. The diversity of tasks is limited, but the vector of such systems and their fusion in one system could be applied to decision making and task solving in a more complex environment.
Processing thoughts is also a matter of optimization. I'm not sure that entire internet hive mind should know that one's lip is itching right now, ups sorry hivemind, it's gone, false alarm. (a lot of people imagine HM as a mean to know any feeling of any person and that those persons exchange those feeling constantly all to all)
So there have to be some arrangements, in this answer I wished to use another word instead lip, but some of the internet rules stopped me from doing that, especially because of possible reaction, which is not what I need to explain that HM idea.
Obviously members will not transmit some stuff, not because they can't or because of rules, but because it's not needed, not reasonable etc - a just common sense of things.
Engaging civilization isn't an easy task, especially if they are space civilization.
Are Borgs a treat for those My star will explode as a supernova. What can I do in order to ensure that my planet survives that? - no, they don't, they will be fed to BH and energy extracted will be by mass, not by how hiveminded they are. That's why we are friend with borgs, they are nice guys, each new year they send presents in form of energy, very deepminded collocutors. So yes, we have a nice time together, even when we are not hivemind like them. But yes, they are not perfect, they drink too much and forget sometimes that we are friends, so meeting them outside our home system isn't such a brilliant idea. Hmm, but maybe it's a game we have to understand ...
In some situations it is not important how smart you are, lack of information, false information as examples. Hive mind might be better at filtering that bs information, IF it is interested enough in that particular time and place to do so, IF it has enough perception members here, which might or might not to represent information correctly and detailed enough to find some inconsistency.
If projection of power of the hivemind in this place and time is weaker than those of their enemies, then they will lose in that place and time, no matter how efficient they are in using their hivemind (sure with optimal actions of their opponents, with can be replaced with more power)
Their(HM) power not in fast decisions, but in long-term strategies, and in the coordination of actions of small and larger groups. Army also can coordinate their group's actions, maybe they need more people for that, so they are less efficient in terms of people, and yes less efficient in each particular individual case - but overall results will be comparable.
On earth army of humans vs an army of hive-minded vampires, skipping the tactics and right to the battle - will vampire have super super advantage, from being hive minded. Yes some, but not really something which can't be counteracted with numbers of humans. Because the main task is engaging as individuals, as corpses, bodies, doing some actions.
It is the same situation as with computing problems - some tasks are suitable for parallel processing, some are not, because of their consecutive nature.
They do not use all their power for any task, it is impossible and impractical. So if one human threatens hive mind, it's not practical to overreact to the situation. 0.1-2 members of hive more than enough for to pay attention to the situation and handle it. As hivemind, they have the advantage to change those peoples each 5 minutes, so this booooring task will be not a burden for members, or by not allocating someone particular, just one of background subconsciousness processes. It is practical to allocate just right enough power for a task. They can fail and underestimate the threat, like a swarm of bees, might underestimate human - they do what they can each, but it's just not enough because of limitation of their hive mind. A hive mind is a system which has its limitations.
If a hive mind has only gunpowder or nuclear weapon and has no acces to space, it can not do much against potential space invaders(that's another long story, but no handwavium, as usual), just because what they do have is not enough to create some resistance - they just can't concentrate enough of power in one place, because of physical limitation of weapon, which is nothing to do with Hivemind. Hivemind'ing does not make them automatically super explorers or researchers - they have to wish to be explorers and researchers or adventurers. If they do not, if as an example they say about the nuclear bomb - yes, it is powerful enough, so now we can chill and enjoy our selfs with the beauty of stars. They can be lazy, they can fall in a state of no doing.
We as humans decided that space too hard and have not much profit or use of us - and where we are now? Not in space, each second we lose tonnes of energy (sun) because of those decisions.
Hivemind's are different, they have individuality, face, manners, habits, patterns in solving problems (individual patterns like fingerprints), preferences, etc etc.
Hivemind of ants will not care about members, a hivemind out of humans might care about members - this difference will lead to the difference in ways which are used to solve problems. Different solutions have different strengths and weaknesses. And not all hivemind's of same species(humans) will be the same in habits etc. Yes, sure it's probably another kind of difference then between humans as individuals or countries, but using them as examples is not the worst starting point to begin to think about differences in hiveminds. Different ancient goods are probably not the worst examples of representing of such hiveminds in human terms.
Algorithms how information is filtered, redistributed between members, how it is processed, processing algorithms, organization - that's what is the essence of hive mind, subject of their evolution, source of their habits, strength's and weaknesses. That's kinda operating systems(OS). It took us at least 40 years to evolve our OS's for computers, but it still far beyond of ease of use and capabilities of those from the Iron man movies, that advanced CAD program or that OS from Mechsuit in District 9.
Improving, changing - it is never ending jorney.