In my opinion .NET is just not there yet. MAUI (the microsoft UI framework and Xamarin successor) does not support Linux or WASM and the main alternatives Uno Platform and Avalonia UI that do support Linux+WASM may be usable for smaller feature sets but as soon as it comes to more complex scenarios or library support, there is so much room for improvement.

Furthermore, the deployment size of .NET is HUGE and NativeAOT is really hard to accomplish as soon as you have a few libraries. Startup times are also not that impressive and deploying for all platforms is really a pain.

For comparison the size of a simple hello world Android App

  .NET > 70MB
  Flutter < 15MB

That may sound like it's not relevant, but believe me, it is.

iOS is supported even better than Android. You can experimentally build applications with NativeAOT for it and there's even native Swift interop[0] bypassing ObjC in the works (the motivation behind it is somewhat elusive to me but hey!)

[0] https://github.com/dotnet/runtime/issues/93631

Is it .NET Core or just .NET? I think .NET Core was renamed to just .NET a while back but it's impossible to keep track of what's what after the many naming convention changes the team regularly makes.