As of 2012:

* Lack of documentation about QML itself (first and third party)

* Unhandled or unsupported corner case like trees that are a pain to implement using recursive rectangles

* Very limited set of widgets (this has improved)

* Ubuntu/Blackberry/Sailfish/PlasmaActive SDK are incompatible

You can hack your way around and get back a good old imperative QPainter and be done with it, but it kind of void the whole point.

In all honesty, Qt has come a long way since 2012 - QML is reasonably well documented nowadays. Controls (the widget library), while not a silver bullet, are good enough (at least on Android, haven't used iOS all that much) so normally you don't need to go as low as your own QPainter or even Rect-s.