Depends on the default Zsh configuration file they provide. When I used Zsh in the past with just the default config I found some Bash 'features' missing. Stuff like reverse-i-search (ctrl-r) and tab-completion. Most of them are simple config changes but they don't make a default Zsh a drop-in replacement for Bash from a user perspective. I think this is also the reason projects like Oh-my-zsh.
