I'm not sure how they've kept up, as I haven't used Qt in some time, but back in the 4.x days the documentation was absolutely stellar. Extremely thorough and almost every non-trivial feature came with code samples. I couldn't for the life of me understand why anyone was using GTK at the time.
My point is not about lack of usability or documentation. Rather that the nature of HTML+CSS+JS lets you do things you can't do as easily in either GTK or QT. In the latter, yes you can easily achieve a certain kind of UI, but not deviate from it, and even formatting/displaying a significant amount of information that doesn't fit into one of the classic widgets can be a bit of a pain.
That depends a lot on what you're used to. It takes me far less time to implement a custom widget that can format/display a significant amount of information that doesn't fit into one of the classic widgets than it takes me to fiddle with CSS to get it to center things correctly ¯\_(ツ)_/¯.
