When a website starts moderating user generated content, its legal status changes and suddenly the company could be held responsible for what happens on the platform. On a platform that's used mainly for sex work (which is still strictly legal in many countries around the world!), that distinction may be all that stands between fines, bans or even jail time, even in what would be considered more progressive countries.
The easy refunds are essential because the payment provider networks will drop you the moment you get too many chargebacks filed against you. The sex industry in general has a problem getting payment from most credit card providers, because people will often do chargebacks when their significant other/family/friends find out about statements on their credit cards, increasing the risk for the payment provider. There's also a certain sense of puritanism in many payment providers, especially American ones from what I've read, and that doesn't help.
As for copying and sharing illicit material, that's a risk that comes with selling any digital media. OnlyFans can't do anything about that. The biggest entertainment companies in the world have tried, and high quality rips from Disney+ and Netflix still reach the web.
In general, OnlyFans is a blessing and a curse. A blessing in that it brings freedom to individuals to perform sex work from the safety and comfort of their own home, without shady back alley deals whatsoever.
However, it's also a curse because popularisation of sex work inevitably reinforces the sexualisation (mostly) of women. Easy websites also have little to prevent abuse from forced sex work, and I'm sure the scum willing to trade human beings and force them into brothels have seen the opportunities here. Lastly, easy entrance to sex work might lead to more people joining in reluctantly because they see no other way to pay for rent or education.
Sex workers are part of any society, have been for all of history and will probably always be in the future. I don't see that as a problem, as long as people do it out of their own volition. However, these websites don't exist in a vacuum, and as long as others stigmatise sex work, joining the industry will come with certain risks for exposure and blackmail. No platform can prevent social issues, no matter how hard YouTube and Twitter claim they're on top of things like misinformation and illicit material. The problem is simply not one of a technical or a business nature.
Having said that, I'm sure there's a market for DeepFake-style face/environment replacements. Deepfake a convincing face from thispersondoesnotexist.com onto yourself, replace the background with furniture from a similar generator, and you could severely reduce the risks that come with selling explicit media. It still doesn't solve the core problem of blackmail and such (people can and will find out, and stalkers are obsessed enough), but it can reduce the risk of those conscious about their privacy despite the nature of their job.