What's most amazing to me about React Native was how quickly we were able to go from idea to product (~3 months working only part-time) using our existing knowledge as web developers. We did have to write a few lines of Objective-C here and there, but for the most part all of the views and interactions fell together the same way a webapp would.
I've seen the Facebook post about developing Ads Manager (though it glosses over some details), but it would be interesting to hear other people's experiences.
https://code.facebook.com/posts/1189117404435352/react-nativ...
Hard to say how much of it was platform-specific, since we only targeted 1 platform (for now). But the large majority of the code we wrote exists in React components or in related JS files, so porting it over to Android should be a matter of decoupling the parts of the code that rely on native features (i.e. camera, navigation, etc).
A lot of that work is already happening in the React Native community so I expect the cross-platform support will keep getting easier.