Harmattan uses Qt Quick Components to create consistent GUI apps in QML. Nemo also use these, AFAIK, and (thus?) Sailfish as well? But what about Ubuntu Mobile?
I want to focus my reading/learning to something that will be applicable for the foreseeable future. I like QtQuick and QML, but without something like Qt Quick Components the fun is quickly over when you want to create a more traditional app. Also, I'm in doubt whether to use Qt/C++ or Python as "backend" for QML. PySide is well supported on Harmattan, but what about the others?