Nix and specifically nixpkgs is IMO very bad at this. It's not a distro: it's a collection of random links that in many cases now only exists in cache.nixos.org. The tarball server frequently doesn't have content, can't represent some content at all (recursive hash types), links have rotted away completely (broadcom driver zips referencing a domain which is now advertising online gambling).

Nix isn't functional: it's a functional core that moved every bit of the imperative part to an even less parseable stage, labelled it "evaluation" and then ignored any sense of hygiene about it.

No: your dependency tree for packaging should absolutely not include an opaque binary from a cache server, or a link to a years old patch posted on someone else's bugzilla instance (frequently link rotted as well).

Nothing has made me appreciate the decisions of mainstream distributions more then dealing with an alternative like this.