The big advantage of using nix modules to run a service is they usually work out of the box, with sane/secure/reasonably production ready defaults. Similar to why you might choose to use a Helm chart to deploy to a k8s cluster. Quality of modules varies greatly, though, depending on their popularity.
Common services usually have undergone some amount of hardening as well, so you probably aren't giving up much in terms of security vs. containers. Again, your mileage varies tremendously depending on the package, so I do strongly recommend reading the source of the nix module of security is a concern.
I also find that consulting the nix source for a service lets me quickly understand the different pieces that go into a deployment - this may not be an advantage to you if you aren't inclined to dive into nixpkgs source on the regular.
Finally, if you are at all bought in to Nix/NixOS then you will greatly appreciate being able to configure your services using module options that have already been created. You can also run docker containers in NixOS, but you'll experience quite a lot of friction if you want to expose service configuration as nix options. Using an existing NixOS module means someone else has done that work for you.
Common services usually have undergone some amount of hardening as well, so you probably aren't giving up much in terms of security vs. containers. Again, your mileage varies tremendously depending on the package, so I do strongly recommend reading the source of the nix module of security is a concern.
I also find that consulting the nix source for a service lets me quickly understand the different pieces that go into a deployment - this may not be an advantage to you if you aren't inclined to dive into nixpkgs source on the regular.
Finally, if you are at all bought in to Nix/NixOS then you will greatly appreciate being able to configure your services using module options that have already been created. You can also run docker containers in NixOS, but you'll experience quite a lot of friction if you want to expose service configuration as nix options. Using an existing NixOS module means someone else has done that work for you.