You can't really compare websites with Qt apps. QML was made from the ground up to allow for building fluid UIs. It is rendered using OpenGL. HTML was never meant to be used for app development, hence the poor experience. Qt and QML is just not native(widgets will look different, etc), but it will perform well. And you can always imitate the look and feel of native OS so apps at least don't look too alien.
I worked for a company developing cross-platform Qt applications(we deployed for Linux and Windows though) and all our clients were pretty happy, so I would refrain from the claim "That's never worked out well for anyone, ever, in the entire history of cross-platform widget library attempts.". My example is just one out of hundreds and hundreds of uses of Qt in industry.
If by 'worked' you mean that you were still able to sell it.
It doesn't fit into the platform and nobody actually likes the result, but if you have a market niche, you can get away with it -- until a competitor appears that actually invests in what their users want.
Qt seems to be working alright for Autodesk Maya and Mudbox. They migrated the UI to it and everyone I've talked to that use these on various platforms don't really have any complaint about the UI (especially since they can use Qt, PyQt, etc... for building plugins).
Mathematica has a Mac-native frontend, and always has. They've used Qt for the Linux/Unix frontend since 6.0, not sure if they're using Qt for Windows or not.
Most of which wouldn't even run on OSX if they weren't written in a cross-platform GUI that dramatically reduces development times.
I'm sure the VLC guys would like to spend their days dealing with complicated video codecs rather than the craziness of yet-another-goddamn-GUI-platform.
