Well, missing packages for one. Nix prides itself on having one of the most compete package catalogs of any Linux package manager, and on Mac it leaves quite a bit to be desired. A lot of functionality has to be hooked in via home-manager scripts that are a lot less stable than NixOS modules, and since your system isn't built/upgraded by Nix you can't write overlays for a lot of Mac software. If you only need the versioned Flake management then it might be an okay option, but I found myself pretty frustrated by it in places that aren't an issue on Linux. I can't comfortably use it as a Homebrew replacement.

Also, my Mac is 256gb which feels far too cramped for Nix. I'd really only recommend it if you're working with 512gb or more.