Not everything is all or nothing. 80-90% reproducible builds are often good enough and learning Nix only to get that last 10-20% is not always worth it. And there are ways of pinning all dependencies with docker if you really want to.

I've had issues with Dockerfiles not building anymore due to changes in the package registry, but it was like 1-2 times out of 1000 of times I used docker.