Xamarin.Forms isn't really equivalent to any (commonly used) Java options. Something like GTK# or WinForms+Monk would. There's also Eto and Avalon "in the pipeline" so to speak.

To me neither Java nor C# does well at WORA UIs but they are at parity imo. JavaFx isn't bad, but it doesn't do anything the C# offerings can't do fairly easily.

I'm actually a fan of Qt for true cross-platform apps these days myself