The biggest downside is it means anyone targeting GTK4 would likely have to target a specific desktop that uses it, since they'll likely be using libadwaita (GNOME), libgranite (ElementaryOS), libclassic (probably future MATE/Cinnamon), and whichever one you compile against... that's what your application looks like.

Well I'm still unclear on this one but if it's as you say then it would…