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.