You can override NixOS's predefined systemd settings from _outside_ using NixOS module options. This allows you to change default settings that are not optimal for your use case, without having to patch NixOS itself, or write your own unit config.

For example, systemd by default permanently gives up restarting services after a few number of tries (e.g. 5), even if you have set `Restart=always`. This is suboptimal for web servers that should recover by themselves after arbitrarily long failures (e.g. network downtimes outside of your control).

On NixOS, you can, from your machine config, set:

    systemd.services.nginx.unitConfig.StartLimitIntervalSec = 0;

This sets/overrides just that specific systemd option for the existing nginx module. On other distros, you often have to resort to global mutation in `/etc` that does not compose well.

We use NixOS for our infra (having used Ansible before), and this ability to override anything cleanly and keeping defaults otherwise made for much easier to maintain infra code and less ugly/surprising compromises.