The FFI is still far better than most of the other FFIs for other Linux-targeting languages. Yeah, and MonoDevelop is not equal to Visual Studio, but it's a damn sight better than almost any other OSS IDE.
Don't know, but QTCreator is a C++ IDE (I think?) which makes it a non-starter for me.
You also can develop just in QML, which is really nice.
In my opinion, I'd say Visual Studio > Qt Creator > MonoDevelop. Qt Creator isn't bad, but the debugging is not nearly as great as Visual Studio. For instance, adding a variable to a watch doesn't always work (as in, variables aren't always evaluated when you'd like them to be) and I can't just type a variable name or expression into the watch window, I can't always view the memory at an object's address, and sometimes I get incorrect values (which could be something with my configuration).