They're already getting decent reproducibility of their curated package set, but some of us want to pin to a particular version of this, especially when creating our own private packages. That's where I'm finding I really want pinning.

The latest release has included "flakes", promoted from an add-on to an opt-in feature and which, among other things, lets me pin my dependencies and the environment I need to build my private projects. I then don't have to worry about going out of sync with the main package set, and I can share private projects in a (hopefully) fully reproducible way, with people using different versions of nix.

https://xeiaso.net/blog/nix-flakes-1-2022-02-21

Guix makes a big deal out of being fully reproducible. It is possible to pin versions, you basically state "I want package X while the guix repository is at commit hash Y". Every dependency of the package will be built and made available at the appropriate version and everything will work out. I used this to roll back fprintd when it broke due to a broken dependency and it worked without problems.