On my last project we started an effort to redo our QWidget implemented UI with QML. The QML version was really slow, we actually ended up buying a commercial license to get access to a QML compiler just to speed things up. And that was for desktop, I'd expect it to be even worse for mobile. Integrating the QML components with the QWidget components was especially buggy. For a 100% QML project this would be less of a problem.

Interfacing C++ to QML is clearly not a priority in the design of the whole system. You can do slots/signals or you can register C++ objects into the QML namespace. Something along the lines of this tutorial: http://doc.qt.io/qt-5/qtqml-tutorials-extending-qml-example....

Overall, I find the whole QML environment to be disappointing and I don't plan to use it again until it becomes more mature.