Is UI elements sufficiently performant to allow you to get away with using unity to make normal ui based mobile apps, as opposed to using swift xcode / java android stuff?
or is software specifically built for making that kind of stuff like react native still way better?
UIElements is almost definitely sufficiently performant itself (though I’d hesitate to say it’s “better” than native code, of course), but Unity itself is quite large (even if much much smaller than Unreal). I would go look into Project Tiny specifically if you’re going to try heading in that direction, as the viability of this really has more to do with everything but the UI, IMHO.
Nope. It would eat battery like a hungry hippo and your users would not be happy. Unity is built for continuous running and i dont think you can force it to update and draw only when a new message comes into the loop.