I like the idea of a declarative configuration, but I find it hard to justify when Ansible has the potential to do the job 99% as effectively.
From my point of view, the strength of NixOS compared to sensible is not that it does the stuff you declared in your configuration. It's knowing that the description is complete and your system does nothing else (because it's basically selectively built at boot). Sure, some options have implications that might not be visible at first glance, but nothing can hide in the long term. You have no such guarantee with Ansible.
Ansible is a good solution, but it doesn't do as much as nix on NixOS.