Re: The missing piece is ...
Have there been any good cross platform GUI libraries?
Qt if you want to run the same UI at the same level of performance on different devices.
As we're talking about C#, Xamarin Forms is okay, but needs more development - I had hoped that when Microsoft bought Xamarin, they'd put development of the Forms and UWP libraries onto converging paths to create a common UI framework, but that doesn't seem to be happening.
The problem with cross-platform is when you need to make something look exactly like the target OS (this is more of a headache for desktop software rather than mobile/tablet apps). You can do a pixel-perfect reproduction, or a native-control mapping, but then the vendor changes everything and you have to repeat the process (Apple is by far the worst offender here - it seems incapable of updating the OS without screwing with the UI toolkit appearance and behavior)