Recent versions of Unity are actually using Roslyn ¹) but they are admittedly running a bit behind on C# language version. The currently supported version is at 9 while 14 came out last month. It's not really a huge issue in practice, though.

With Godot 4, the big difference between Godot and Godot .NET is that the version with NET support does not build to web and mobile support is 'experimental' ²). Also, they are two completely separate downloads and editor binaries, which makes switching languages decidedly non-trivial.

For a 2D game, if you can live without building to web, I'd pick Godot. Otherwise, I'd pick Unity.

¹) https://docs.unity3d.com/6000.2/Documentation/Manual/csharp-...

²) https://docs.godotengine.org/en/latest/tutorials/scripting/c...