It's not because of the nix language, It's not because of the CLI, it's because everything is scattered, you have to consult many places to find out how to do things with Nix, here is an example:
Usually, when I need a new complex program, like Steam, I first check the system-wide configuration [1], the wiki [2] and the package list [3], if I just want it on my user, I need to check if Home Manager has an option [4], if it doesn't, I can try using the "home.packages" option. Now, if I need to override something on the package, I need to remember how to do it with [5] [6] (while checking the source code for the package in parallel to find the options).
And then sometimes, on very rare occasions, I need to fine tune something with the nix language, so I need to check the builtins/lib docs [7], but some builtins are not there, so I need to either use nix-doc [8] or find the docs inside the code-bases [9] [10] (they are split between both repos)
For me, this is one of the main pain points of using Nix / NixOS that needs to be solved.
[0] - https://github.com/shiryel/nixos-dotfiles
[1] - https://search.nixos.org/options
[2] - https://nixos.wiki/wiki/Steam
[3] - https://search.nixos.org/packages
[4] - https://mipmip.github.io/home-manager-option-search/
[5] - https://nixos.org/manual/nixos/stable/#sec-customising-packa...
[6] - https://nixos.org/guides/nix-pills/nixpkgs-overriding-packag...
[7] - https://teu5us.github.io/nix-lib.html
[8] - https://github.com/lf-/nix-doc