Then it is fairly bad as a tool, you don't wan't to spend disproportionate amount of time learning to use the tool when the goal is to just use the tool and forget about it (ie. using Nix for more deterministic builds).
It took me a long time to learn git. When starting out, I could only follow some rote commands, and wasn't able to work my way out of mistakes.
Or, tools like Vim or Emacs are tools with steep learning curves, but which provide a powerful user experience if you take the time to learn them. -- Although, you can certainly get by using other editors like VSCode without having to put time into them; but I wouldn't consider vim or emacs bad tools.
Developers commonly work with packages. (e.g. when installing toolchains, or dependencies for building some package). Having a tool that can so expressively and powerfully deal with packages like Nix can, I think if it's got some rough corners or needs some effort to use, it doesn't discount it as a tool.