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).