I'm very happy with screens implementation. It will stick to the size of the first client and you override it at will in any other client or if you want to update the size (maybe you changed the size of the window).

I don't want to detach my desktop just because I want to check current status on my phone. Maybe I have two sessions on two computers and I, temporarily for a few commands, want to go fullscreen. With tmux I have to detach the other client(s), go fullscreen, and when done I have to reattach the client(s). This for something that takes a command to adapt and again recover in screen.

It is just an inconvenience, nothing huge, but big enough to obstruct my flow.