The imperative parts of QML is javascript. So lack of type safety in larger apps can start getting really annoying, especially once the code base starts to age and not all corners of the code base gets tested every day.

Other than that, beginners to the codebase tend to break property bindings without realizing. There are a few tools to catch this at runtime, but would be nice to do this at compile time.

Also not all of the "Desktop widgets" were readily available in QML in Qt 5 (not sure what the status in Qt 6), and you tend to roll your own Table Views etc.. and those ones need even more work to work like native widgets, supporting keyboard navigation etc...

Overall Qt5/QML was fun to work with, but I did miss a few bits that were readily available on QWidgets.