I'm a zsh user, and I disagree on it not being much of an issue. It is very easy to inadvertently rely on all manner of zsh-specific behaviour in scripts. Short form logic syntax, builtins with different arguments, internal stat/attr/datetime/etc manipulation, my personal favourite $RANDOM behaves differently to bash in subshells, and much more.

zsh does have the emulate builtin available to help with some basic things, but it is still easy to produce scripts littered with zsh'isms.

However, I do use zsh for personal scripts. In fact, I go so heavy on zsh'isms I know I'd need to rewrite things to share them if non-zsh users are an issue. It is easier to keep track of things that way, instead of tripping over the little easily forgotten quirks.