You're not the only one, many developers have mentioned (for at least a couple months now) that this release of GameCenter has a multitude of issues. I myself am not able to submit any leaderboard scores (even though no error is thrown) As someone fairly new to Apple's technology, I probably would never again consider Swift/GameplayKit as a choice for a serious game. Maybe one day the core features will be corrected after Apple hires QA engineers?

Correcting my previous reply, it's really not that buggy. But the lack of large community backing makes it pretty difficult to find out what's wrong when you're stuck. But the library does provide a nice way to make games in Swift.

Yes, it will be. I can say in 2020 there are still too many bugs to consider this a usable game framework. Shot myself in the foot and all I can see is how greener the grass is on Unity's side