I love C# and I used it extensively for a long time, but I think Copilot shines with JS/TS and Python because of the massive amount of code published in open source projects.
Not that I find it as useful as most people do, but there is a difference...
It's also not that great with go, but surprisingly (to me) still better than C#.
A bit ironic since StackOverflow began life as the C#/.NET QA site - and nevermind copilot is a Microsoft joint.