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.
That’s not a KDE specific issue, it’s Qt defaulting to the default ugly old Plastic theme.
KDE uses Qt, but unlike GTK, Qt isn’t made for KDE. Qt have their own defaults that are generic and doesn’t make any assumption about running on KDE or Gnome. And Gnome doesn’t bother providing any configurations to Qt, because they live in their own GTK bubble where nothing else matters. Adwaita just happens to be the default GTK theme regardless.
Ideally, your distro would take care of setting a saner default Qt theme. There’s even a Breeze theme for GTK, so they could set it to that and everything would look nice and uniform.