The problem I’ve had with bad themes (e.g. black on black text/icons) seem to stem from apps using system/fallback colours, e.g. not defining it’s own list background because it assumes it’s always going to be white. But I can’t say if it happens more often with Gtk or Qt… Most recently it happened both with GIMP (Gtk) and FreeCAD (Qt)
In all fairness, I’ve had some really dicey problems on Linux. I think the most difficult problem I’ve encountered was when I bought a USB soundcard which only worked in legacy mode, but using Wireshark and the USB audio class specification I managed to track down the bug in Linux’ usb-audio module, so now I’m technically a kernel contributor :)
For me the difference is that when I get a problem on Linux, it usually tries to provide me with the information I need to figure out what goes wrong, and due to its open nature it tries to make it easy to fix things. Also the majority of the time, Linux is working flawlessly. Windows on the other hand, is plagued by bugs and annoyances that show up on a weekly, if not daily, basis. And when issues happen, they come with little information and are often impossible to fix yourself since the OS is locked down.