The problem is that bash syntax for anything beyond the basics is awful and error prone. It's really hard to make significant progress while still being bash compliant.
I don't know specifically what you're referring to, but zsh has a bunch of extensions to bash syntax (or rather, takes advantage of a lot of "empty space" in the language) which make it possible to just ignore the bashisms and use more sane features when scripting specifically for zsh, while still being able to trivially convert scripts with most bashisms.
