I see nothing especially incomprehensible about Nix, the language itself pretty simple. About the only thing I had a bit of an issue with is that I kept forgetting that functions only take a single argument in Nix. But other than that it is very nice to work with and has all the syntactic sugar you want, string interpolation, sets/map, sane multi-line strings and all that. Syntactic sugar is an area where Scheme has basically nothing to offer, everything needs calls to functions with long names.

For most common uses you barely even have to care about the language, as it's just some JSON-like data with shell scripts in between that do the actual work.

The language very much generates complaints because it is gives you no real help understanding what argument should be passed to what function and how to get that argument. Error messages are dreadful because there's often a million functions your object has to go through before it produces an error. Dynamic typing and lazy evaluation don't seem to mix. Some kind of compile-time, presumably gradual, type system would really be an incredible improvement, so that type errors were generally eager.

Incidentally, this is one of the things that some of the people involved with the blog post in the OP have set out to fix. A few years ago, Théophane (author of the blog post) started an experiment to add gradual typing to Nix, which eventually became the start of Nickel: https://github.com/tweag/nickel

Today, Nickel development continues at Tweag, but now they have someone else, Yann Hamdaoui, driving the implementation forward. (I assume because he's a domain expert in programming languages.) Producing helpful error messages is one of the major goals of Nickel, as a potential successor language to Nix. All of the language design decisions are documented as GitHub issues on the repo, so if the technical side of this kind of problem is interesting to you, you may enjoy browsing that.

It appears that Nickel is very close to a usable state for early experimentation. You might be interested in cloning the repo and messing around with it!