> Qt

Arcane build system. I mean, I guess it technically supports CMake these days, but I have never been able to get anyone else's Qt project to build without much gnashing of teeth.

Emulated native widgets try for pixel-perfect, but tend to feel wrong somehow.

> Gtk

Outside of a Linux/Gtk native environment, Gtk applications are awful. Take GIMP on macOS, for example: it's had window focus issues (export dialog getting lost behind the main application window) literally ever since Gtk on macOS dropped the XQuartz dependence. And that's the flagship application for the toolkit.