Are Xamarin tools really the holy grail of cross-platform app development? What are the hidden gotchas?

What about MonoDevelop for the desktop? The Windows and Mac download packages are at v4 but Linux at v2.6. Are Xamarin slowly abandoning use of GTK# ? Is this because of GTK 2 v 3 fragmentation?

Should we be supporting C++ and QT given that QT now supports iOS and Android? Given that some developers are scared of C++ what about C# / F# with QT QML? Are there any bindings which are maintained and tracking QT updates?