I've used Qt, Swing and JavaFX, and found all of them absolutely terrible. Qt might have at least led to some good results, but it's fairly bad to use and deploy. Especially if you want to use the latest Graphics APIs, which worked pretty badly back when I last used Qt in 2016. I much prefer Electron, or imgui for C++. I'll happily sacrifice some features so long as I dont have to use Qt.
> Qt might have at least led to some good results
You seem to focus on the outcome while what interests me is the programming model. Say what you want about object-oriented programming, the problem-space lends itself very well to it. In Qt/Swing/… components are just classes whose behaviour and appearance are determined by function overrides, taking as parameter event or graphics objects. Just like that, you cleanly solve everything that "webcomponents" stands for: interface contracts are determined by class constructors/parameters, customisation is just class inheritance with member override, look&feel customisation is plain calls into the graphics API primitives, layouting/responsive-design is taken care of at the just level of abstraction, and all of it takes place in the same language and ecosystem/stdlib (no HTML to CSS to JS references to keep in sync and mental overhead).
React reproduces some of this model, but with much more technical and mental complexity, bringing trade-offs of its own. I'm not calling this easier to work with, simpler, or easier to debug. It certainly gives more creative options, but only because the ecosystem is broader (and full of supply-chain risks).