Well, you can almost surely write entire apps in QML if you take the C++ entrypoint for granted.
Just a couple examples: you can do Bluetooth discovery scans in QML without writing any C++ [1], you can read from ~20 sensors without writing any C++ [2], you can write complete 3D scenes declaratively in QML [3], and the list goes on.
> But it is Swift, which (at the risk of stating the obvious) QML is not.
sure, and I agree that Swift has some better features and a better foundantion than JS on which QML is based upon
> And you can't write an entire app in QML.
This however is not true. I've worked on multiple pure QML apps so far (well, if you don't count the auto-generated main.cpp). Remember that you have access to a complete ES7 JS engine which allows for a lot of stuff.
There's something to be said for being able to use one language for all things.