@pixelfed Tumblr claims that they block the subset of nsfw they do, because otherwise credit card companies would refuse to do business with them (and so they'd find it difficult to be pair, even if advertisers were totally fine with it). I don't know enough about the context to know whether I should doubt this claim. Assuming it's true, that's a problem that might be shared by pixelfed instances of nontrivial size: their users might find it hard to donate money to cover operational costs.