I know it's a shameless plug, but if you want to convert any Java bytecode with less semantic differences than scala.js and full reflection support you might want to give https://www.defrac.com/ a try.
100% code reuse across all platforms is unachievable. What a lot of do today is complete horseshit relative to the state of the art, but different platforms have different ui conventions and following their rules can't be abstracted away into some library, obeying these conventions requires some intuition about human psychology.
By embracing the JavaScript platform, the scalajs folks are leaving the playing field open as to how to deal with multiple platforms. Maybe we can hook up scalajs to react and react native, there seems to be a bit of hype around that.