The question is why would you want to have non-nixos servers in that case. If the goal is to have super-fast automation, you still need to unify your setup around something common. I see more benefits in unifying around NixOS than in unifying around Ansible/Pyinfra, as the latter require me to specify my infra in terms of low-level OS- and distro-specific package managers, that would eventually define the same lack of "OS agnostic portability".