...for sexual oppression and all kinds of abuse.
It's a hierarchical power structure which is isolated from interference by outsiders. The dictators (biggest, oldest) decide who gets to interact with whom and when and how and can easily restrict the flow of information to and from the household using simple control tactics, and such decisions are usually afforded a unique respect that they do not always deserve.
A child is essentially locked into a state of extreme dependence on one or two parents (depending on who's around), and this is usually viewed as "better" or "natural" for the child regardless of the parents' style of governance. There can be ritual spankings; frequent loss of control leading to physical lashing out; belittling, hurtful, or psychologically manipulative gestures or speech; any form of abuse; neglect including the important but oft-forgotten denial or withholding of physical affection, or an inability to provide that; any number of serious issues and if the kid doesn't speak up since it's Daddy who's making them feel like shit and he "puts the food on the table, you ungrateful little brat", it will (more often than not) continue to cause negative consequences yet never addressed.
Domestic issues are bound to happen in any type of close-quarters living, but it's the excess power that enables parents to isolate and confine and almost pathologically avoid addressing said issues that turns them into chronic "baggage".
By contrast, a child who is able to more-or-less select their role models and guardians by choice has a much fairer shot at avoiding negative influence or a straight-up malevolent upbringing.
If someone's regularly bringing you down, you should have the freedom to stay away from that person rather than a sustained obligation to please them.
Children rarely have the former and OFTEN are stuck with the latter.
This is just the tip of the iceberg.
I suspect that children prosper when they have prosperous relationships. Of course it's best if they can feel happy and secure outside the home as well as inside. If inside-security isn't possible, then outside-security becomes a MUST. With larger family units, my guess is you end up with better inside-security on average and better access to outside-security. You have options.
Aside from far less agitation and stress, richer and more varied interactions for better learning, fewer opportunities for problems to fester and suffocate, less toxic hierarchy, and an inherently greater amount of personal freedom (more independence from an earlier age with more/better adult support): open-concept families are probably more likely to be accepting of soft consensual incest, although I suspect the nuclear family format probably leads to more of it.
(Both small and large family formats can create the important sense of belonging; but I suspect small formats are especially prone to creating a very unhealthy sense of ownership over one's subordinates.)
I don't see the nuclear family format as evolutionary "progress" - just one approach of many that became a default template in certain cultures for reasons that didn't really have much to do at all with child-rearing. I could be wrong about that, but either way I would much rather see our ideas of family open up a lot more because from my experience: isolated, held-as-sacred, claustrophobic, and dictatorial is exactly the kind of environment where very dark things can transpire (and be forgotten).