what is the sweet spot? I'm faced with developing a fairly simple thing and it would help to know a little more about learning curve ( I use react ), downsides, etc.
Small to medium sized apps (not games) where you can sacrifice some UI polish to get them delivered quicker and on both platforms. Have a look here https://facebook.github.io/react-native/showcase.html If you know React, learning React Native would be pretty easy, plus you can use many of react libraries like redux, etc.