Device support is a nightmare from a game development perspective if you're coding in C++ and using the GPU directly. Figuring out why the black box shader compiler is crashing on a particular device is not fun.

Perhaps screen sizes was a bad example of fragmentation (though, because the range is so much wider than on iOS, keyboard avoiding stuff can be unpleasant). Better examples would be subtle device-specific bugs. Like, off the top of my head, touch handling being handled differently for views on the Galaxy S5, making side swiping impossible. I can't confirm if that's RN specific or not, but I do know that I spent tens of hours on Android specific bugs that just were not present on iOS.

And perhaps RN was the culprit for some of these things. But definitely not all.

Same experience here. The API is the same, so what exactly is the problem? There's an occasional device-specific bug, but I usually don't spend much time on those.