I honestly don't really get why people have such an issue with the Brint to Brienne transformation. Now, I'll admit, I didn't interact much with Brint, as I was only interested in knowing him platonically, but from my limited interactions with him, and my interactions with Brienne, I got the feeling that the transition from one to the other was believable, reasonable, and not really a problem at all from the perspective of either. Yeah, one is more dominant, the other more submissive, but honestly is that really necessarily such a defining personality or identity trait? For some people maybe, but from the platonic impression I got of Brint, that doesn't seem like a big part of his identity or anything. They both just seem to do whatever feels natural and enjoyable to them with their body when it comes to sex, what that might specifically be doesn't seem like something that bothers them, and while I can't relate to that, I don't see an issue with that either, nor do I think it should be seen as insufficient reason for why Brienne is ok with her transformation.
I'll admit though, I might be missing something, there might be a compelling argument that would change my opinion, but I'm not familiar with it at the moment.