Potentially it was a mess, but the guys building it contribute to nix so I don’t think that’s to blame. Dev environments are okay, but that’s not the limit of nix - it gets used for build servers, ci servers, linting, replacing stack cabal yarn etc, creating docker, hell I’m pretty sure I saw nix creating nix
TFA points out using nix as a reproducible build environment - which it is excellent at. Create a shell.nix in your repo, and every dev uses the same exact tools within the comfort of their own machine/shell. Docker is much more painful for this kind of local dev workflow.