Coming from a Java/Kotlin background, I found C# to be really nice, but I had some issues with Xamarin tooling. I used Xamarin Studio and for the most part, it worked fine. Sometimes, there were some random crashes and project build errors that mysteriously resolved themselves. I also needed pie charts per the customer requirements and the best pie charting library I could find had licensing issues, so ended up implementing the pie chart myself which wasn't that bad.

To compare, I would say the Flutter tooling is really good, compared to Xamarin where sometimes I would not be confident with the tools. Also, with Flutter, I can share the UI layer entirely, compared to the Xamarin project where we only shared some plain business logic with written purely with C# without Android/iOS SDK dependencies. I know about Xamarin.Forms, but have no experience to compare it with Flutter. At the time the project team didn't want to use Forms, but I don't remember why.