While you might have SOME kittens adjust to leaving under 8 weeks old, the majority of behavior/social problems in cats can be traced back to leaving mom and siblings under 10 weeks old.
During the time between 4-10 weeks old, kittens learn a lot - how to use the litter pan, learning to eat solid foods (most kittens WILL nurse up to 8-10 weeks old - 6 week old kittens are deprived of this and may show sucking problems); learning how to play with other kittens/cats and learning when to bite/scratch, etc.
When you take the kitten away too soon, your kitten uses humans as playmates and will bite/scratch a lot more in "playing" because they have not learned this is wrong. They also don't really interact with people very well under 8 weeks old.
Good breeders teach kittens how to behave in the house, how to use the scratching post, kittens are TOTALLY weaned from mom and have time to properly socialize kittens with older cats, dogs, kids, other humans AND give them most of the shots before leaving - that is why you won't find breeders letting kittens go till 12-16 weeks old!
My rex kittens were never allowed to leave till 4 months old because I taught them a lot of house manners before they left (also were used to being groomed, nail clipping, and bathing. They were much more socially developed when they left - no problems in hiding in the new home, interacting with strangers, etc.
Shelters can't do all these things and so they have no choice but to release them early and hope for the best. That's why a lot of shelters will have "foster" parents that can help socialize kittens and puppies till they are placed at 3-4 months old.