They provide the same immutability guarantees

Not as easy to have your entire system introspectable, sharable between devices, and under the same VCS. NixOS is declarative first and foremost, and all other benefits fall out from that; immutability is just one of them.

NixOS is a hack that shoehorns existing FOSS software into what an OS should really be. I doubt we'll get anything better for the foreseeable future, except maybe Guix.