"Learn once, write everywhere". Can't speak about performance comparison vs using Qt/.NET/whatever but the whole idea about React Native is that today, you can write a
native
app for iOS, tvOS, Android, web, macOS, Windows, and a few others, all using the same language and framework. You still have to understand platform specifics like UX guidelines but you use the same technology to build a fully customizable, and fully performant, native experience. It's sort of like using C++ but way more developer friendly.
The cons:
- an extra layer of abstraction
- need to write a plugin if you want to surface a native functionality to RN if it's not yet supported
- no visual UI editor like in native tools (Qt/VS), although you generally won't need it with React Native's code-first and instant refresh development workflow
- you need to be comfortable with React Native / JavaScript, obviously :-)