Question Combining standard app and Unity app for good UI

Unity is great for interactive 3d content on mobile devices but it does not work well (as well as a native app) for UI heavy content or web-based/web-view content, e-commerce etc. as the resource hungry, fps-based nature of the engine and text rendering do not compare to a web browser or web view.

So to build an e-commerce or other ui heavy app with a Unity 3D section do people use something like Xamarin for the core app and add a Unity app section or can Unity only be integrated with native iOS and android apps, not those built with a cross-platform solution?

Any experience of, or disagreements with, the above would be a big help!