Another good contender is Xamarin (it is even getting merged into .NET 6).

C# is a really good language with great tooling (my subjective opinion, I like C# more and more).

C# also can be used to write:

• cross-platform CLI apps and services
• crossplatform mobile apps
• crossplatform gui apps (projects like Avalonia, Uno Platform)
  • MS is working on MAUI which should be crossplatform GUI framework)
• web backend and frontend
  • Blazor (C# running in WASM) looks interesting, though I thin it is not yet ready to replace JS for me)
  • ASP.NET Core is extremely good, actually one of fastest frameworks out there (according to some benchmarks)
• Games (Unity framework)
• Machine Learning (ML.NET)
• IoT

The only problem is “Microsoft” - as a linux guy I still feel a bit worried/uncertain about MS. Who knows what they decide again.

When C# was still only for Windows, I would never consider it, but nowadays, the benefit of one language and be able to write for so many platforms, it is definitely a strong contender to at least keep an eye on…

Wonder if somebody compared it with Flutter.

I think, if I would start to learn mobile development, I would learn Kotlin right now. When I remember correctly, you can build iOS apps with Kotlin.