I haven't used C# that much in Godot, but the speed boost would help several of my projects. Not to mention, game logic and other code written in C# may be more easily transferable to Unity, and potentially other game engines since C# uses static typing.
It is definitely something to play around with in the future!
I think there is C# support for Android and WebGL now, though last I remember reading, iOS still is not supported. Though, it's been awhile since I've checked, so maybe things have changed.