Well for Dan people said for years he's too touchy with the kids and he has a weird foot fetish. For the other two producers they were convicted of assaulting children, one who was on the show, and Nick and Dan tried to cover it up as much as possible to avoid bad press.

If you actually watch the documentary they also talk about 2 pedophiles who were on his set at nick and convicted during that time and one pedophile who was convicted from a disney set.

It's less about Dan schneider specifically (although he does seem to have a lot of unprofessional, sexist, and inappropriate mannerisms) and more about the need to increase protections for the physical and mental well being of child actors as a whole