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.
> Yeah, I eventually realized that nix-darwin is only good for managing the list of homebrew casks.
If that's really all you use it for, and you already use Home Manager, the Homebrew module for Nix-Darwin works fine in Home Manager with some small tweaks. I use that with a custom, per-user Homebrew prefix to keep Homebrew out of the way except for the task of installing casks.
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.