Qt is obviously very solid, definitely "production quality," while QML is solid in the sense of not buggy and the UI looks/feels/performs quite well on Windows/Mac (haven't tried on Linux). But QML can be slightly to considerably irritating to use because it's not always clear or intuitive how to accomplish something or why something isn't working the way I expect, and I usually don't find many examples/tutorials when I turn to Google. That said, it's far, far more enjoyable to build an app with QML than with HTML/CSS/JS; I'm way more productive and the result is an actual application rather than a weird facsimile.
