I'm more familiar with GTK, but can't you still make it look like anything you want pixel perfect?

I think the real issue is you can't bring all of your javascript/html/css libraries and tooling over.

I think you're correct.

This is a total non issue with Qt Quick, but I think complete UI look control is a waste of time and effort. It’s wheel reinvention where light brand accents would suffice.

Qt and Juce can be customized to look however someone wants. I don't think it is really so difficult to make images for buttons, change palettes and make sure everything is anti-aliased.