From my experience, QML timer is all but accurate betwen plataforms.
I have experienced significant speed diferencies between linux and windows, wich makes the game characters move with very diferent speeds on diferent plataforms. This can be a problem and make the game unplayable.
Iinitially I tought that Timer was just a QTimer exposed to QML, but has AFAIK thet are not related at all.
QTimer seems to provide a much better accuracy, so what I did was to created a new c++ class that uses QTimer, and expose it to QML. In the end, I got a MyTimer qml entity with similar Timer sintaxe, but with much better results.