Is mainly the language. Is not just weird like APL , that insanely make sense. is weird.
I don’t find it weird compared to other functional languages.
That's not the compliment that you think it is...
(I say this as somebody who deeply loves the concepts in most functional languages, and loves the concepts in nix, but recognises that the developer-experience of both is about as pleasant as reading somebody else's code-golfed perl)
It isn’t a compliment, just a fact. Of course it all depends on what you are familiar with, but that is a fact about you, not Nix.
The concepts in functional languages are not fully realized in any multi-paradigm languages. So you can admire all you want, but don’t knock it until you’ve actually tried it.
I don't find Nix the language particularly weird. But pick an arbitrary .nix file, see a bunch of stuff passed to the derivation, and try to find out whence it comes, and what types / attributes are expected there. The only definitely working solution I know is a full-text search over the file tree.
Now compare it even to Haskell, to say nothing about e.g. Typescript.
That... is all on the language. In fact the main trouble now that you spelled out is the major blow against nix: is terrible at modularity!
https://guix.gnu.org/ might fix the language by replacing it with a real one.
Guix is a nice idea. But the few last times I read about it, users uniformly noted that it's soooo slow in man cases. Nix can be slow, but most of the time is very fast, it just has to check a lot of files.
I say this as a daily user of Nix.