NixOS just doesn’t let you do that in the nominal case, most of /etc consists of symlinks to a read only partition that is managed by nix - it is actually more difficult to do one-off scripts or config changes via files than it is to do so via nix, at least nominally - there are of course software that has it’s own special config format or that keeps its config in a database - but those get snapshot(ed?) and backed up anyway.

Imo, nix is more finnicky but more of a complete solution than ansible.