It's true somewhat though, but it's a problem is not easy to solve for Python.
The problem is that Python supports C extensions, and for example the psycopg2 package has an extra dependency on a libpq which it expects to have installed on the system i.e. if you were using a docker you would do something like yum install postgresql-devel otherwise it will fail. Python won't install it for you and it doesn't even have a way to relay (except for error message) that such package is needed.
Many people actually get stuck with this and won't know what to do, but quick google show that package is needed they install and everything works. Nix is functional and doesn't have state so you can't just invoke nix-env -i postgresql and now everything will start to work, that behavior would actually ruin reproducibility. Nix instead expects that the C dependency is also provided otherwise it will refuse the build.
That's what OP meant where he said Nix exposes weak build systems.
Anyway, there was significant progress in making things better in python, it is still not awesome, but it is very close. Actually the biggest pain point right now is to setup a build environment that's convenient to use.
I recently created a template that shows how to do it: https://github.com/takeda/example_python_project
It uses pip2nix and it can automatically figure out all python dependencies, the packages like psycopg2 need an additional entry like this: https://github.com/takeda/example_python_project/blob/master... because PyPI packages don't specify any system dependencies.
