Reproducible is key here. I remember trying to manage servers with Ansible, Chef and Puppet in the past but was always disappointed that trying to spin up a new server, or move a service between two usually failed. This is because service S1 forget to declare dependency D, but it happened to work because S2 needed the same dependency and declared it. Or simply that in the past S1 declared D but the config was "cleaned up" and it wasn't realized that D was still needed. (Or it wasn't needed at the time but then needed again, but you didn't declare it).

These types of problems are very rare in NixOS (although I do occasionally see it with the module system, but at least this is config dependent, not system-state dependent). If I apply the same config I have a very high confidence that I will get the same system. I lost my desktop's drive the other day and I was back fully functional within an hour. Most of the time was spent logging into Firefox, Email and various websites rather than installing the system. Managing servers is similarly easy, I would have a hard time moving to something else.

Intrinsic state (like user data or application databases) is still hard, but at least that is the only real concern. Better than dealing with state and config.