OMZ provides opinionated defaults and doesn't expose you to all of the choices that the typical zsh config process does. Which is the exact opposite of the Arch Linux mantra of being aware of every choice that you are making in which software runs on top of the kernel.

I use OMZ mostly because of dotfile inertia. There is some slight weirdness (vaguely remember something about negative/exclusion globs) compared to vanilla zsh, but it isn't bad.

I think it's oh-my-zsh that they get mad at. Maybe because it's too 'magical' for them? I don't know.