QGraphicsTransform3D and QGraphicsTransform duality
Is there a reason for separating QGraphicsTransform3D and QGraphicsTransform? They seem to have nearly same APIs but they don't share a common ancestor besides QObject. This makes transforming QtQuick3D items (requires QGraphicsTransform3D) and regular QML items (requires QGraphicsTransform) side-by-side difficult and awkward.
QGraphicsTransform3D being named QQuickQGraphicsTransform3D in c++ is another source of awkwardness.
IIRC, the first one applies to the old Qt3d module (which was never officially released) while the other to the QGraphicsView and friends part of Qt Base. If you are interested by the Qt3d module, you should rather check the wip/newapi branch.
Hope it helps