Developing an app with React Native on iOS is a very good experience. On Android (which I understand is still very new), everything feels a little more complicated and I find myself writing a lot more native code than I would like.
So far I've had to write Java code for: Geolocation, Alert Dialogs, Sharing intents, Facebook login (this one wouldn't be in core anyway) and Google Maps (I would have probably written my own wrapper for this even if it was in core because I'm doing some very specific things). What's frustrating is that these modules don't contain a lot of code...
I would suggest prioritizing getting Android to feature parity with iOS over adding new features, starting with:
  • Geo Location
  • Net Info
  • Dialog
  • Intent