As a fan of Nix, and someone who expected a lot from Conary/rPath back in the day, I can provide at least a partial answer: these distros are great as long as you use their packaging tools for everything. Which is simply not practical. There's a reason Nix and rPath never took off, and it's the same reason developers use virtualenv and rvm instead of system packages. They don't like being forced to use a single packaging tool for everything.

Docker doesn't have an opinion about how you package things. It only cares about the resulting changes on the filesystem. So you are free to use the best tool for each job.

By the way this means you can use Docker and Nix together. I would love to see that :)