I have used all three! I started with Server then went to CoreOS running Kubernetes and settled on NixOS which I have been very happy with for about a year now. I run about 25-30 services all using built in modules.

Regarding security, if you are using well crafted modules on NixOS, there should be good systemd hardening in place. That being said there is no reason you can’t just use containers on NixOS.

I also find deploying NixOS far superior to butane/ignition used by CoreOS/Fedora. I use nixos-anywhere and can deploy my entire server in a few minutes without manual intervention.