There’s a couple of absent macOS-specific packages, compared to homebrew. So I have `trash` and `pngpaste` declaratively compiled-from-github in my nix config, but that's the only two I'm missing compared to brew. (That said, I never dealt much with brew-cask, so if you have a host of GUI packages from that, you may be missing more)
If you're on an M1 Mac it will use x86 binaries by default, running under Rosetta. The situation is improving - I use almost all of my packages in native aarm64-darwin - but it still requires using bleeding-edge code.
The learning curve has been pretty brutal, but I really like it for both managing my system-wide (well, user-wide) software, and for pinning to explicit package versions in a large ruby/javascript project.