When did you have this issue? My account is tied to my Google account because on launch the Go servers were completely inundated and the account creation was just constantly failing but using a Google account allowed you to skip that step and start playing.
This was the first month of Pokemon Go years ago. I haven't heard of it being an issue lately but I also haven't needed to create an account in a very long time.
I think the first time was around the spring 2017. I could create an account, use the credentials to log in, but trying on several days the game never started. There was some "please wait" kind of screen and waiting for hours didn't help. With a Google account things worked right away.
And a bit over a year the same thing happened. New Pokemon Go account -> log in -> no game. With Google account has been working since.
So, my experience is two tries in the span of two years it did not work.
This was the first month of Pokemon Go years ago. I haven't heard of it being an issue lately but I also haven't needed to create an account in a very long time.