A while ago I entered a hackathon visualization challenge using Qt painter for rendering. I expected C++ and Qt would slow me down but the rendering speed and expansive drawing primitives were valuable! They greatly reduced my iteration time and increased fun.
Today I never create new frontend projects with Qt. I may use Qt creator with cmake for backend components, but the frontend is HTML and JavaScript with libraries like React, Three.js, D3, and bootstrap. The experience is still cross platform, but also one step away from launching as an internal portal. Distribution this way is much easier, but the tools are so flexible you need more diligence to keep the code maintainable over larger teams.
I struggle to see where you would prefer Qt for frontend code if you don't need a native app.