I use both GNOME and Plasma and various apps from their ecosystems. When using Plasma, GNOME apps look out of place, as they hardcode their Adwaita theme, but they don’t suffer from contrast issues and are perfectly usable. When using KDE apps on GNOME on the other hand, the contrast is terrible, the apps look very ugly and are barely usable to the point I wish they hardcoded their Breeze theme so they would work as well as they do on Plasma. I wish KDE apps were more resilient when it comes to theming, so they wouldn’t break completely when installed on the “wrong” desktop.
the kde apps are made theme agnostics specifically to not interfere with the way other desktops/distro want to theme them, and the gnome desktop is specifically made to not natively handle user themes so as to not interfere with the way apps are supposed to look like, mix that into a bowl and you get ugly kde apps, which one is in the wrong is for you to decide*1, but at the end of the day you need another app to handle your qt-theming since gnome doesn’t natively support it.
spoiler
*1 it’s the gnome devs of course