I know both Win32 API and MFC rather good, and can say that none of them is bad.
Same for me. However, using Win32 API directly for bigger projects is just insane. Small and medium-sized projects yes, but use it for something like a web browser or a word processor and you are majorly f***ed.
wxWidgets itself is really great and since it is older than ROS you can see that it survived several OS and GUI versions, with respect to the supported OS/GUI.
Oliver