In my opinion tmux has two shortcomings. The built in session manager is very limited(so I am using my own script instead). And the ability to save and restore sessions is achieved using two plugins that can take some time to wrap your head around and make them work.

Zellij on the other started with the goal to be as user friendly as possible and basic tmux features(such as detach) were implemented very recently. There are too many elements on the screen, and each actions requires you to go into a "mode" which feels like unnecessary overhead after getting used to the way the modifier/prefix approach works in tmux. And I am more then fine with modes(I use neovim and emacs with evil keybindings) but when it comes to terminal multiplexer or window manger I feel a modifier provides less friction.

When its all said and done and tmux is configured I feel I am getting a more fluent experience. Unlike vim and emacs I don't see a lot of distinction between tmux and Zellij and I don't see a many reasons to switch at the moment.