I too have struggled to remember which third party sign-on I used (or if I used a native sign in), so now I avoid them every time, too.
They're literally only convenient if I want to have an account that I'm happy to 'throw away' or, to accidentally create duplicate accounts for the service.
For anything where I'm actually paying, they're a nightmare. Oh, did I sign into this with one of my google accounts? Was I crazy enough to use facebook? Or which of my emails did I use?
I suppose that's a huge assumption, but that's how I would do it if I was developing against them. That said, it doesn't help w/ the "Hide my Email" or the default icloud.com email addresses people don't realize they're using.