zsh isn't a perfect super-set of bash (there are some differences in syntax, provided built-ins, &c.), but it does have equivalents for most of bash's functionality. Off the top of my head i can't think of anything useful that it's missing. People may have subjective preferences for e.g. readarray over parameter expansion, but the functionality is all there. I suppose it doesn't (and probably never will) fully conform to POSIX, if that's important to you