I mean it can be better for privacy if you think about Google/Facebook loging. But it will prevent adding all third party login services, potentially even ones

I mean it can be better for privacy if you think about Google/Facebook loging. But it will prevent adding all third party login services, potentially even ones that are more privacy respecting than Apple. Also there are cases where a "sign in with <particular provider>" is the only option that makes sense because you really want to integrate with the API of this provider. Take for example a "sign in with GitHub". Or in case of services correlated, take for example Instagram where you obviously can sign up with a Facebook account. I'm more for letting the developer choose what it prefers for authenticating the user and not having a authentication system that gets imposed by Apple.