It's actively developed but there's no mature cross-platform UI (Xamarin.Forms is mostly mobile and has many issues). Using something like MvvmCross combined with native UIs on each platform is the best option for now.
Re:What Makes .NET Core So Special?
by giancarlostoro on Friday November 09, 2018 @12:30PM
(#18416143)
The only use I ever saw of it by others was to make mobile apps for both Android and iOS so I think it will stay there worse case.
I'm feeling more fearful that they acquired Xamarin just to destroy it.