Complexity has both an essential and an accidental part. Nix is not (yet) as friendly as it could on the second part, but simply due to the problem domain it will have quite a bit of essential complexity, no matter what.

Package management is a complex problem and nix is the fist software that actually solves it, instead of just hiding it in the closet (e.g. docker-like solutions). If you want to deal with packages correctly (and I think we all want to), then this is the price.

Although it will improve on the accidental complexity part, e.g. currently many language ecosystems/software is not built with nix in mind.