Earlier quoted context omitted.
this installer comes with an uninstaller https://install.determinate.systems/
Thanks for the link. Apparently there were issues even as recently as 4 months ago, specifically conflicts between the determinate installer and nix-darwin though I'm not sure they are intended to be used together. https://news.ycombinator.com/item?id=40158558#40159195
As it happens, /etc/nix/nix.conf is one file where such a potential conflict can occur. As an optimization, I think it will (sometimes? always? idr) replace a file if the existing one and its intended replacement have the same contents. The defaults /etc/nix/nix.conf in the mainline Nix installer and the defaults for /etc/nix/nix.conf in Nix-Darwin are the same, so it doesn't complain there. But the Determinate Nix Installer ships a nix.conf with different settings, so Nix-Darwin refuses to replace it, to avoid clobbering your (the Determinate Nix Installer's) settings 'changes'.
There's no real compatibility issue, and the Determinate Nix Installer is doubtless intended to be used with any Nix-based software, including Nix-Darwin, Home Manager, etc. Nix-Darwin installation is just a bit hairy, as it's unfortunately always been.
NixOS is a better experience in that respect, and you should consider running NixOS in a VM if you want to get a feel for that kind of systemwide, declarative Nix configuration without worrying about installing it against a foreign base system. (If you do learn your way around nixos-rebuild, you won't need the Nix-Darwin installer anyway.)