I have software people use up on pypi and I'd love to switch to uv for personal use to benefit from the improved speed but I'd need some guarantees that things work EXACTLY how they work on pip or that I could run them concurrently. If I put instructions up for users to "just run pip install xxx" I need to know that if they see any errors I can see those too for debugging/troubleshooting.
It does not work EXACTLY how pip works, big differences are covered here: https://docs.astral.sh/uv/pip/compatibility/
Some of these are uv following the standards while pip is still migrating away from legacy behavior, some of these are design choices that uv has made, because the standard is underdefined, it's a tool specific choice, or uv decided not to follow the standards for whatever reason.