The description I've heard on Software Engineering Daily is that React and React Native are more "learn once, use everywhere" than "write once, run everywhere". That is the important productivity goal is related to programmer knowledge, skill, and ability rather than lines of code.

Keep in mind that React and React Native are from Facebook and reflect its needs not necessarily those of a single developer or small shop. My understanding is that for Facebook, the problem React and React Native address is allowing feature based teams rather than platform based teams. For example, the new "like" button could have Web, Android, and iOS developers all sitting together sharing a common language while still developing on somewhat divergent code bases.

That's not to say that one way data flow isn't useful. But there's still still no silver bullet.

Good luck.