Flutter's edge over these technologies is faster development cycles (very fast build times, sub-second reloading without losing state, very rapid ability to update code/GUI after deployment to a testing device, and very fast render times).

Xamarin and Qt are fine products, and I know Xamarin has been working on some improvements over deploy times, but it still can't touch what Flutter does there. And if you know C++ and C# already, Dart is a breeze.