I can't describe how many hours I and people I have helped have lost to save a few miliseconds of machine time.

For example preferLocalBuild ignores builders and --max-jobs per #3810 giving the user the impression that flags are being ignored.

Having such exceptions completely change the way Nix works makes it also really hard to debug binary cache issues - I've gotten lots of reports of why is binary cache not substituting.

Refs #3686