There's the rub. If you don't know anything about the native platform then you "don't know what you don't know". The NativeScript demo shows how to pull in a native ObjC function (a string related method I think), but if you start with no background in the native libraries how would you even know when you could be leveraging a native library function instead of trying to recreate a sub-optimal approximation in JS. But if you have the foundation and know the native platform, why jump ship to JS at all? And then I'm right back where I started :)
I'm a (native) Android and JS developer. The primary reason I'm excited about React Native is that I don't have to go through the tedious compile-install cycle for every little UI change. React Native will make prototyping/iterating on Android as easy as it is on the web. However my reasoning assumes that there will be almost no friction in terms of creating complex views in React Native. I'm curious to see if that pans out.
No, that's the beauty of it. All you need to do is shake your phone and tap the "Reload JS" option. Those JS files are served by some sort of a server that starts up on your dev machine when you first deploy the app. See https://facebook.github.io/react-native/docs/tutorial.html#d... (search for "reload js").
Different cross-platform tools have different approaches, but as a concrete example take Corona SDK. You need no native platform knowledge and your app runs inside the Corona runtime, which makes some difficult things easy but some easy things -- third-party SDK integration, install source tracking -- difficult. And if the creator of the cross-platform environment has chosen not to support certain things, you're out of luck (and you're probably always going to be lagging even for things that are supported). On the other hand, you have only a single codebase to maintain and single-click builds for a lot of platforms.
As for Cordova, we spent some time testing if the performance was sufficient when building an early version of Recent News (https://recent.io -- v1.0 just released three days ago!). We concluded it was not. Boot times were slow, the UI felt very non-native, and overall performance didn't meet our requirements. Now that was a while ago and Cordova may have improved, and hardware performance certainly has improved, so maybe it's a better option today.