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.