I've always thought ‘multi-document interfaces’ as we used to call them are an anti-pattern. I have a perfectly good window manager; why does every app need its own incompatible, usually inferior window manager built in? (Mind you on mobile I very much don't have a perfectly good window manager, and indeed can't even open multiple instances of most apps…)
To throw gasoline on the fire: this how I’ve always felt about tmux. Why use an incomplete in terminal windowing system when I can just have multiple terminal windows open managed by the superior OS window system. (That said I know tmux is sometimes the only option and then it makes sense to me)
tmux (and screen) are incredible assets for remote sessions, both for continuity across dropped shells and multi-shell activities when the connection process is tedious (multiple jumphosts, proxies, etc.)