To me, the worst part is when GTK introduced what's known as "client-side decorations"[0], because that means that I can't use my normal window manager decorations.
If I force them on with something like gtk3-nocsd[1], then although I get my normal window manager decorations back, sometimes the client-side decorations have actual important widgets in them like menu buttons. When that happens, I end up with windows that have both kinds of decorations, and it looks weird.
For me this means that it's not nice to use apps like Meld and Remmina.
Just let me use my own WM's decorations please!