I think the primary downside people find with QT is the language support. People want cross-platform, native UI (i.e. not GTK) in a statically typed language higher level than C++. Even Go versions require a long compile time and don't work w/ the msvc toolchain and (IIRC) QT quick and QML are a bit hamstrung if you want to do advanced things with controls.

I would love full QT with a modern, high-level static language that is not hobbled on Windows. .Net may be the first to get there with a quality cross platform UI (Mono's WinForms are a bit hobbled too).