That's "common knowledge" but I wonder how true that is.
I have a feeling NSFW attracts more problems from a "moral", political and pressure groups perspective than actual fraud and that's the main reason companies don't want to touch it.
The rate depends, but take OnlyFans: They cascade (I.e. spread out) payments across at least 3 providers, and automatically rotate between them to avoid any of them getting a rate too high.
Seems like quite a bit of logistics if it was not really an issue.
That doesn't really make sense unless they have a way to flag higher-risk charges and route them to a processor that is more risk-tolerant (and usually more expensive). Eg. I could see segmenting the charges for the first month or two into a higher risk bin, and then moving recurring customers into low-risk bins on processors with cheaper rates.
There is a difference between straight up porn and fan-service (OnlyFans) and the adult artwork and erotic educational content on Gumroad. I would be surprised if these suffer from the same amount of chargebacks.