Places that use functional language for their code base don't have problems picking it up. It is especially big in Haskell community, because Haskell is also lazily evaluated.
People might wonder why not use language that more people are familiar? The reason for it is that purely functional and lazily evaluated language is basically behind Nix's biggest strengths. Purely functional means for the same inputs(architecture + source code + dependencies + configuration options + etc) you should get the same output, there's no mutable state the languages always starts the evaluations from a known state.
Why lazily evaluated? That makes the Nix only evaluate things that are necessary to obtain given derivation.
I think when people talk about Nix being confusing they really mean NixPkgs. Which is like a stdlib of Nix. The thing is that Nixpkgs in a constant flux people who work on it work on different components somewhat independently. Also often they realize better ways of doing things, and often the documentation doesn't keep with it. I think that Nix needs some kind of mechanism for literate programming where documentation would live together with the code and would be updated at the same time. There are some effort for example like the NixOS options page is generated from the source, but that's just just small part of Nix and only applies to NixOS.
I also would love if nix would standardize on how to pin nixpkgs. I recently discovered Niv[1] and it makes reproducible builds so much easier. I think Nix also needs some opinionated tooling to create dev environment for each supported languages, currently people need to do it themselves, and it takes a lot of research.
I think Niv + lorri + direnv make dev experience extremely enjoyable. For example a python project set up that way. Once you enter the directory, you have the exact version of Python installed that has all the dependencies (of course with locked versions) and all extra development tooling that developers need with exact same versions. All you need is just run the application and it just works every time. If somebody broke something you can easily use git bisect to find when and why it broke.
Nix fails to document though how a new user get to that point, and desperately needs help there.
