Insisting that there can only be one input type for the entire session rather than one per window. If you are using multiple languages which each require an IME, Gnome's interpretation is completely broken. I had to stop using IBus because of it. It is possible they have changed their mind since then (several years back), but I haven't followed it. Incidentally, it was also the thing that meant I had to stop using Gnome. Before that I was a happy Gnome Shell user :-(