iOS and Android UX guides have completely different approaches to the logic of user interaction. AFAIK, Xamarin does not allow to have different layouts generated from shared code. So, even that components are native, their placement is alien.
> Xamarin does not allow to have different layouts generated from shared code
That's true, but noone says or recommends that all the code is shared. Typical Xamarin architecture and good practice assumes sharing the business (or domain) layer, and coding the views separately. In terms of reusability it's still superior in comparison to writing both versions natively. Obviously the level of reuse you can achieve that way very much depends on the nature of the application.
I also don't get your point about how lame Xamarin apps look. Xamarin apps use native layout components (could be wrapped with lowest common denominator abstractions in case of Xamarin.Forms, but it's optional), so they're 100% capable of looking the same way your native app would.