As an example, I recall trying to build a package for psycopg2 (it wasn't in nixpkgs at the time, and even if it were, for one reason or another we couldn't directly use many packages from nixpkgs), the most popular Python library for interacting with a Postgres database; however, that required me to write packages for a bunch of C libraries that I didn't understand at all, including fishing random header files out of the Postgres source code. No one in our organization including our Nix veterans could figure out how to build it correctly, and it ultimately caused us to move away from Nix (it was among the last straws).
Besides, just because it didn't work for us doesn't imply that there's a problem with our personnel (by several industry key metrics, our devops capability is excellent), but rather that the effort required to implement Nix successfully was greater than the work required to use other tooling. We could use C++ instead of Python for our organization, but just because we don't doesn't imply that our organization is incompetent; rather that Python's tradeoffs are more appropriate for our problems. In effect, Nix creates more problems than it solves for us, which is unfortunate because the problems it solves are really important problems, and many of the problems it creates are utterly unnecessary (add docstrings, use a more familiar expression language--maybe Starlark [https://go.starlark.net], provide and document 'escape hatches' so users can use system dependencies where it's prohibitively difficult to hermetically package things, etc).
