See what people are promising when they say "declarative" is a dream. You just tell the computer the final state you want, and the language does what it needs to do to get there.
The problem here, is that in order for that to work, the language has to know how to get where you want it to go, and give you vocabulary to tell it that. That "how to get there" and "vocabulary" has to be written in a (gasp!) general-purpose language.
So now, you come across a situation where you need the declarative language to do something it doesn't know how to do. So now you're forced into the position of helping create the declarative language. And as it turns out, creating a declarative language in a general-purpose language is a lot harder than just using a general-purpose language to tell the computer how to go where you need it to go in the first place, because you have to delve into an existing codebase which is inevitably giant.
I've begun to refer to this anti-pattern as "Configuration-Oriented Programming" (COP).
I'm being a little facetious here--I've not looked into Nix enough to have any educated opinions on it. But it sounds like my outsider impression that Nix is a COP might not have been too far off.