With Nix it seems like the technology is there. It works. It has a superior model compared with other systems. However, it lacks a corporate sponsor or large community to make it "professional-grade". Canonical and RedHat make their money from people depending on their packages being continuously up to date. Debian has an enormous community ensuring that stable is stable and secure for the duration of its life. Nix doesn't have either, so it is up to the users it does have to be reactive.

That said, I think the benefits of Nix are real. The existing configuration management systems do not have the ability to fully understand the system. Nix does, because everything about the system is contained in the configuration file. As a consequence, you can up and down grade servers without fear of breaking everything. You can check to see exactly what changes between different versions. There is much more control and more assurance that the nodes are in the proper state.

One part of your comment which I do not understand is:

> When you can stand up a fresh VM using Ansible in a few minutes, the value of being able to roll a server back easily is hard to justify.

Isn't this also true for Nix? You just launch the image and push your desired configuration? If you mean time to write a new ansible playbook/role then in my experience, although I am fairly productive with ansible, I would not put it at "a few minutes" from nothing to a new service. Maybe you are talking about the learning curve? Nix definitely seems steeper so a clear win for ansible on that front.