The idea for x-risk defense bunkers would be to create a "civilizational" refuge, much larger and much more secure (airtight, unusually deep underground, etc) than any military/government bunker. It would be designed to function for years or decades (for instance, it would probably contain a small nuclear power plant), and contain a population of hundreds or thousands of people -- plus all the critical industrial tools and scientific knowledge required to rapidly reboot civilization when the surface became habitable again.

First of all, this means that the logistical and design challenges are pretty significant... you'd have to worry about recycling resources like water and air, and maybe even growing food. It would be a lot like architecting a sci-fi "generation ship" or a Mars colony.

As for social dynamics: On the one hand, I'd hope that a population in the hundreds to thousands would make this much easier. The bunker might be crowded and stressful, but you'd be still living in something like a normal society, where you could go to different places each day and hang out with a variety of different social groups. You wouldn't be crammed into a tiny space with just five other people for months on end. Also, even if things went REALLY poorly and people started killing each other in the bunker, most people would still survive unless they did something suicidal/terroristic like destroying a piece of critical life-support infrastructure. A little social/political drama would probably be fine.

But on the other hand, the whole point is to create something capable of maintaining and restarting civilization all by itself. Would a thousand people sitting in a bunker have the skills to maintain industrial civilization? This seems pretty hard, even if the residents were well-chosen to create a diverse mix of skills and expertise. I guess they could always emerge from the bunker and return to subsistence agriculture, but that seems like a pretty precarious situation for the last 1000 humans on Earth!

So, I think the danger is less "the crew would go mad from isolation and kill each other", and more "our civilizational bunker might keep people alive but slowly fail at the task of actually preserving civilization, and then humanity could peter out afterwards".