nix has been around for 18 years. If the documentation is still sparse after all these years, it's not a problem that will be fixed, ever. Same for explaining concepts.
Proper errors are a huge and poorly understood problem, so I'm inclined not to judge those too harshly.
I don't think it is fair to judge a project for the number of years its been around, specially for something that is really novel and innovative. Nix/NixOS is not a new web framework that most of concepts are really clear for everyone working with Web. It is a brand new concept for Operating Systems, however most people don't have a clue about system immutability and reproducibility and why this is so hard.
Of course, this doesn't excuse the state of the documentation that should be better. It is just that nobody ever documented this kinda of things before, so it is difficult to do it better.
Nix and NixOS just started to get more known and become popular from the last ~5 years or so, and I think this made the whole community aware that documentation is an issue and it is getting better and better (but maybe not at the pace most people wanted).
Proper errors are a huge and poorly understood problem, so I'm inclined not to judge those too harshly.