Uh, what? I've not used Nix with python (so I don't know if there's something particularly bad there), but most of the complexity comes from handling non-python

Uh, what? I've not used Nix with python (so I don't know if there's something particularly bad there), but most of the complexity comes from handling non-python dependencies of python packages (which different people have different needs and hence opinions on, and so seeking consistency there is like expecting all the linux distros to merge). At best there are two major systems that people will talk about (pip+venv and conda), and the reality is conda isn't a python package manager, but a more general system (and every other python system you could name is built on the pip+venv system).