I don’t know if anyone noticed, but Visual Studio’s startup screen featured a link to Avalonia a few weeks ago. I guess that’s some kind of indication that Microsoft might throw some support behind Avalonia.
If this technology weren’t such a fledgling technology, I’d be putting my eggs in this basket rather than Xamarin Forms. Xamarin Forms looks alright, but it just smacks of the issues that web developers face in terms of cross platform compatibility issues: i.e. always having some quirk to screw you over because of some slight difference in control behavior on different platforms.
Avalonia looks like you could convert a Silverlight/WPF app smoothly over to a cross platform app, but more importantly, the UI would be the same across the board. I think that’s what is really missing from Xamarin Forms.