A quote from the author of Ruminate GUI..
So about MonoGame support, I have found MonoGame very difficult to deal with. The second you vary from the basic functionality offered by XNA things start to break. Resizing behaves differently between platforms, form handles are handled differently between platforms, and lots of other things require a deeper knowledge of the underlying technolgies (OpenTK, SlimDX) and (MonoGame) than I have and am willing to learn at this point.
It's unfortunate that the author feels that way about MonoGame because he is giving up multi-platform support in favor of only supporting Windows.
Of course, there are a few take-aways from this..
MonoGame is awesome, but it still has a lot of little bugs causing developers to shy away from it. The MonoGame team should pay close attention to feedback like this and focus energy into fixing those bugs in the core framework before working on new features (IMHO).
Ruminate GUI is open source. If the author is not going to port it to MonoGame, perhaps someone else could do it. I'm sure the author would be okay with that.
It highlights the need for a good GUI framework that works with MonoGame.
Interestingly, I actually started working on a project like this about 12 months ago. It was coming along very nicely, but at the time there wasn't any interest in it and I found it too difficult to maintain. Unfortunately, I've also re-factored the code into my game engine, so it's no longer just a stand alone GUI library.
There's a great screenshot on this early blog post:
If there really is some interest in a GUI library like this, I might consider reviving the project.