The Nix package manager also runs on macOS. Think of it as a cross platform Homebrew. Your other objections apply to all package managers, and not just Nix.
I'm principally against abstracting package management across platforms, because experience has taught me that the software management subsystem of the target platform is the best system for that platform. I see no point in this abstraction since I understand that the best packaging format is the native format, and I've no problem mastering the native format.
I couldn't care less about NixOS, as I'm a UNIX (IRIX, Solaris, illumos/SmartOS, HP-UX) guy and always will be. I run Solaris 10 and SmartOS on my infrastructure, before that I ran IRIX and HP-UX as well on it.