Misogyny and dehumanization of sex workers mostly. I'm pretty negative on the porn industry generally because it is very exploitative, but OnlyFans is probably the most ethical form of sex work. Completely controlled by the workers, there is profit sharing but just the normal level of revenue sharing a platform holder does, similar to non-sex work platforms like YouTube or Google play/App store.
I mean, there is no way to know for sure that there isn't some scary pimp in the background holding a gun to the sex worker's head and stealing all their revenue, but that's highly unlikely. There have been some stories about people forced into desperate circumstances and turning to OnlyFans as a last-ditch effort to make money, but at least they are self-employed and see the vast majority of their revenue and have control over what they do and don't do. I wouldn't be surprised if that is much more attractive to many people than working a low wage retail job where you don't get to set your own hours and are exposed to coronavirus.
The biggest negative is likely the amount of harassment they receive, but that is true of all sex workers and I would hope OnlyFans has moderation tools to help mitigate that. I've never used the site personally but from what I understand, if you care about consuming porn ethically it is probably the best way to do so.