This may be your personal experience but it is used successfully and happily by many in a variety of domains way different to "boring OOP back-ends". No popular general purpose language is confined to a single scenario the way you describe (even Swift and Kotlin are expanding). C# lets you do things like writing a competitive Redis implementation[0], advanced physics engine that beats Jolt[1] or a game engine that does not rely on C++ or C[2], cross-platform applications[3] and so on and so forth. It is possible to successfully do both high level FP style approach for business domain modeling and very low-level data crunching that is competitive with systems programming languages.
My personal opinion is a little more blunt than that. In my world C# developers are typically people who are attaching wall anchors with the hammer function on their power drill because they can’t be bothered to pick up an actual hammer. It works, but it’s not a good idea. Like I said. I think C# is a good language for a lot of things. It’s just that it’s often also the wrong language for specific things. If I were to use a “power drill” for everything I’d personally pick Typescript but it’s not like that would really be a better idea.
This being HN, however, most people here are more prone to actually go pick up the hammer.
[0]: https://microsoft.github.io/garnet/docs/benchmarking/results...
[1]: https://github.com/bepu/bepuphysics2
[2]: https://www.stride3d.net
[3]: https://avaloniaui.net