But one big difference between Xwt and Xamarin Forms is that Xwt supports Linux via GTK (GTK1, GTK2 & GTK3) whereas Xamarin Forms does not.

Xamarin Forms also requires different code for different targets, such as:

   // Android
   Forms.Init(this, null);
   var androidFragment = new MyFormsPage().CreateFragment(this);
 
   // iOS
   Forms.Init()
   var iosViewController = new MyFormsPage().CreateViewController();
 
   // UWP
   Forms.Init(e);
   var uwpElement = new MyFormsPage().CreateFrameworkElement();

Xamarin Forms Gtk# Backend - https://github.com/xamarin/Xamarin.Forms/pull/1174

It's all floating out there right now but maybe by end-of-year ("Q4 2017") there will be something solid. This uncertainty promises enough to keep people like me from evaluating alternatives.