For the Creator, even if you exclude the things that interact with Patreon to do the benefit-management bits, you have to also do any KYC/Identity verification bits, re-setup account transfers.
Then you need to reach out to all your supporters and convince them to move over. They're going to lose some percentage of those people -- how much is going to depend on the creator and the fanbase. Some people are just going to ignore it, every single one of them though are going to re-evaluate that math in their head as to whether they really want to continue supporting that person.
Patreon also has major brand-awareness. Someone telling you to move over to SubscribeStar, Liberapay, Buy Me a Coffee or whatever might get a "This seems like a scam" type reaction.