For a desktop application widgets are better than QML. QML brings things to your UI that a desktop application shouldn't use. QML is the right way to do phone applications, but there are good user interface reasons to not use most of the features QML gives you on desktop applications, but to use those features on phone applications.
Not that you cannot build good desktop applications with QML, but there will be a lot more work for questionable gain.
Please give me a single reason what QWidgets does have that qml doesn't, from a user perspective.
> lot more work
I would argue the opposite. Show me a none buggy QWidgets application that features nested collapsing menus, with animations. That being said, you can still create a modern app with fancy animations in QWidgets. The beautiful Telegram QtWidgets desktop app is the best example, it's just more work....
That you cannot do those animations is a feature! Animations are useful in places, but they also make for hard to use user interfaces. Widgets where designed in a world where user interface experts sat real humans down behind on-way mirrors and watched how they used applications: those experts had long learned fancy animations look great but typically result in it being harder to use user interfaces.
The point of QWidgets is not to create a "modern app with fancy animations", that's what QML is better suited for. It can still be done, QWidgets supports theming and all that, but that's not what it does best.
QWidgets is for making well integrated, "boring" desktop apps. For nested menus, or any menu for that matter, use QMenu. If your OS is fully supported by Qt and the standard system behavior is to animate the menu, it will be animated in your app.
I will say that the KDE system settings application uses QML for its sidebar and it behaves terribly when resizing the window (takes around a second to resolve the layout after resizing). I haven't seen the same behavior with QWidgets programs.
Not that you cannot build good desktop applications with QML, but there will be a lot more work for questionable gain.