You may take a look yourself at the repo while I will try to describe it below.

I use nix-darwin on top of nix installation to manage most of system settings and build.

I am not quite sure now, but I think, I remember having to do xcode-select --install, can't quite remember if that was part of workaround for issues or part of normal process. Oh, and there were some discoverability issues with bin directories, I had to work around them by adding extraInit to zsh that includes binaries into PATH. This might be based on my poor skills in configuring everything, though.

Some of the packages needed are installed from nixpkgs, but some are installed by brew based on configuration.nix so these have to be upgraded in brew and don't get upgraded upon rebuild (it looks like the intended way would be to wipe them on each rebuild, but that would take longer)

I develop in Rust, so I also use rust-overlay instead of rustup to manage Rust and nixpkgs-cross-overlay for cross-platform builds. Build scripts also create docker containers descripted with nix in our case.

There were regular issues with Nix installation upon macOS updates, requiring me to either add back Nix to zshrc or completely reinstall it (the latter was not necessary, likely), but those seem to be fixed, as of now.

I think you will have less trouble managing reproducible machines with Nix, but I also think that it will require a bit more than just copying config and running Nix. There will also be issues of installing some software and packages locally, that will allow the developers to break builds across the machines when one have something locally that other doesn't, but this can be mitigated by processes and getting them acquainted with Nix, at least it did it with me 🌚