Parity with Unity/Unreal is very much a non-goal for MonoGame. They're very different abstraction levels with very different purposes.

I know, the point was about any FOSS engine, and the overflow expectations of pushing them as alternatives.

Out of the pan, into the fire.

The biggest advantage Unity has over alternatives like Godot and MonoGame is the ecosystem around the engine, not the engine itself. Things like the asset store and the wealth of tutorials out there. It will take time for the ecosystems around open source engines to match or exceed that of Unity, but the more people leave Unity the faster that will happen.

Unity and MonoGame are not really comparable. Unity is a game engine that is also an IDE, whereas MonoGame is simply a set of libraries.

Or to give an example: Unity provides a decent workflow for artists. XNA doesn't.

An engine that also targets consoles and VR headsets, uses C# as the main language, has an AOT toolchain for it, thanks to HPC# subset also allows for low level engine code in C#, and naturally the graphical debugging tools for profiling and GPU.