Like (iirc) systemd-resolved has `enabled` which is false by default but then gets silently turned on if you use systemd-networkd. How are you supposed to figure that out without reading the source?
But I think this also stems from the fact that the default state of nixos is "a general purpose linux system" and so instead of just starting at 0 and adding the things you need, you have to mix adding and removing things which IMO makes things much more complicated (except maybe for newbies to linux who don't know what's necessary for a running system).
nixos-rebuild repl and then you can inspect things like config.services.resolved.enable or :p options.services.resolved.enable.definitionsWithLocations
With a default config you start with a console, systemd, dbus and some things to make it boot. There is barely anything.
For other distros, that kind of behind the scenes changes are triggered by package installation and ad-hoc commands. It's not always easy to figure out exactly what has changed, especially after the fact. This, in turn, makes changes difficult to revert.
NixOS is much better because you can inspect the changes after the fact. You also know which code to look for, which is a luxury. If the code seems too much, there's the repl to help. Changes are also much easier to revert.
But I think this also stems from the fact that the default state of nixos is "a general purpose linux system" and so instead of just starting at 0 and adding the things you need, you have to mix adding and removing things which IMO makes things much more complicated (except maybe for newbies to linux who don't know what's necessary for a running system).