I see your point about there being different ways to install a package, but I think I can clarify a bit by explaining how I use NixOS.
If I'm running a package on a server that means I want to install it declaratively, so I find the name of the package in Nixpkgs and put it in my `configuration.nix` file. I'm using flakes, but the configuration is exactly the same, I just put the package in the output section of the flake. Any instructions you see to install a package just boils down to finding the name of the package. To me this is as simple as finding the name of a Debian package and running `apt` to install it.
If you want additional features there are other optional ways to install packages, but these are features other distros don't offer, so if you just ignore them then there's no extra complexity compared to Debian for example.
If I'm running a package on a server that means I want to install it declaratively, so I find the name of the package in Nixpkgs and put it in my `configuration.nix` file. I'm using flakes, but the configuration is exactly the same, I just put the package in the output section of the flake. Any instructions you see to install a package just boils down to finding the name of the package. To me this is as simple as finding the name of a Debian package and running `apt` to install it.
If you want additional features there are other optional ways to install packages, but these are features other distros don't offer, so if you just ignore them then there's no extra complexity compared to Debian for example.