I'm only aware of examples where it's the fault of the packages - i.e. they specify dependency version ranges that don't actually work for them (or stop working for them when a new version of the dependency is released). No tool can do anything about that on the user's end.

> Or if you run on a different OS and the dependencies are different there (because of env markers), your requirements file won't capture that. There are a lot of gotchas that pip can't fix.

The requirements.txt format is literally just command-line arguments to Pip, which means you can in fact specific the env markers you need there. They're part of the https://peps.python.org/pep-0508/ syntax which you can use on the Pip command line. Demo:

  $ pip install 'numpy;python_version<="2.7"'
  Ignoring numpy: markers 'python_version <= "2.7"' don't match your environment

There are a lot of serious problems with Pip - I just don't think these are among them.