If all you need is basic splits, sessions, and some simple templates/layouts (and like the convenience of knowing that tmux is widely available, and often installed by default) then you're fine to stay on tmux.

Zellij can do things like floating windows, contextual keybinding guidance (helps learn everything that can be done), and a more complex layout schema. You can disable all the UI eye-candy and switch to tmux-style bindings too.

It's worth trying out. I use both so that I can still function on systems without it.

A ton more conflicting keybindings.

I switched away from tmux a year or so ago due to one crash I kept getting, but thinking of going back. Really miss the simplicity.