I sit beside a coworker who wrestles with tmux daily.

He likes it, but it also frustrates him; it crashes semi-regularly, it sometimes locks out input when one program is spamming one of the tiles, and due to it intrinsic nature you can no longer select-copy large blocks of text with the mouse if you have any vertical splitters.

The lack of block text select-copy is alone enough to keep me away.

Most of the other benefits can be gotten with screen, with better reliability and performance. I use several customized .screenrcs for various tasks, to get started up quickly every reboot. But I don't run screen in every terminal because native scrollback buffer is too valuable.