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.