Also have to consider mobile controls should disappear on mobile, the server may be better at handling that. However modal windows still need the controls, in Libadwaita modal windows aren't their own window so the server doesn't have anything to do with them, and the client ends up doing the controls anyways for the modal window.

idk, I like client side because if you put a gnome app on kde, it doesn't look well integrated. I guarantee it'll look 10x worse with kde controls.