I've been developing with Xamarin since 2013. Primarily Android for a commercial application (30,000 lines of VB.NET Windows Mobile converted to C#/Xamarin). I also converted a small app that I developed for US Army to B4X and Xamarin Forms. Xamarin Forms worked well for this app in that 95% of the code was implemented using XAML with very little platform specific code. The B4X version also went well with most code in the shared class with conditional compilation handling differences. I especially like the Hosted Builder as Xamarin requires a Mac (which I do have) and uses XCODE on the Mac to create the IPA. Xamain iOS is tightly tied to Xcode which requires that I constantly install new versions of Xcode which can take hours! Bottom line: I much prefer B4X to Xamarin and NSbasic. I don't mind maintaining multiple layouts for a small project.
On the topic of watch development. I've created a proof of concept using Xamarin Android Wear. Pretty easy, but very heavy (13MB). So far unable to build a B4A version as the watchface library isn't compiled for AndroidX.
Thanks B4X.