I got a test box set up with nixos and a config that runs all of my services. I wanted to test the declarative rebuild promise of it, so I:

1. Filled the services with my some of my backed up data (a copy of the data, not the actual backup)
2. Ran it for a few days using some of the services
3. Backed up the data of the nixos test server, as well as the nixos config
4. Reinstalled nixos on the test box, brought in the config, and rebuilt it.

And it worked!!! All serviced came back with the data, all configuration was correct.

I'm going to keep testing, and depending on how that goes I may switch my prod server and nas to nixos.