There's an element of truth to the "poor documentation" complaint, because there are tutorials etc out there that are out of date, incomplete etc. But the official nix project has excellent documentation.

I think the problem is that nix is just hard to learn. It's really different from most package managers, but people expect to look at a few examples and just pick it up intuitively. It's a whole different paradigm, with a custom programming language that also has a different paradigm. (Though Haskell folks eat it right up... no surprise there.)

It's gonna take time and effort to learn it, and there's just no way around it.