That should scare people.
NixOS on the other hand, builds everything in sandboxes that only expose the requested dependencies. Builds are almost 100% deterministic. The cherry on top is that your system installation is simply a package consisting of all other packages and configuration symlinked together, and you replace your entire system in one go (atomically, by writing a symlink).
If the build fails at any point along the chain, you get an error and your system remains unchanged. You can totally switch from one major release to the next (and back) without hassle.
I worked at a place where they'd spin up a new gentoo box for a dev with stock "emerge foo" and let everything run up.
Then 3 weeks later, they'd do it again for the next dev, and they'd have many diff versions.
I know they weren't "doing it right", but they were doing it the default way they learned, and it caused a lot of problems.
Thanks for the info/clarification though.
Finally someone understands what really annoys me when I say that docker isn't 100% reproducible if you aren't using version pinning or something similar.
If you have 2 developers, and one of them does a build the next day, you could have them with two different versions of a package when the version went up.