For me, nix the language itself is not that bad. It's all the magic happening in nixpkgs and the tight coupling there with the user interface (configuration.nix, shell.nix, etc.).

In NixOS/nix, you are essentially extending the nixpkgs source code with your own configuration to build a system, which requires good familiarity with the source. This would be like having to write C code to install packages on another OS. It's a questionable design choice but it does offer a lot of power.