My personal take is that you should always develop for the platform you are targeting.
It is not so much about weather the UI is drawn so that it looks native and it is not about performance (although important). It is more about what you can and cannot do natively. For example, in iOS/Mac certain things are not easy to do because the UI components that you are extending are not designed to work in they way you want them to work. This may look like a limitation but I see it as an enabler because it forces applications to have more or less the same behaviour. This is what makes AppKit/Cocoa etc. a better platform for development on iOS/Mac.
It is not so much about weather the UI is drawn so that it looks native and it is not about performance (although important). It is more about what you can and cannot do natively. For example, in iOS/Mac certain things are not easy to do because the UI components that you are extending are not designed to work in they way you want them to work. This may look like a limitation but I see it as an enabler because it forces applications to have more or less the same behaviour. This is what makes AppKit/Cocoa etc. a better platform for development on iOS/Mac.