I am not sure and I have used systemd through NixOS in particular for a good half a decade now. While I have not tried to fight it, it does come across as being somewhat opaque to me, but I can generally deal with it and have not gone out of my way to “read a book” on it. It did annoy the hell out of me about a month ago when I had this “conversation” with it:

    > sudo reboot
    [sudo] password for ninjin:
    Failed to reboot system via logind: Connection timed out
    > sudo halt
    Failed to set wall message, ignoring: Connection timed out
    Failed to reboot system via logind: Connection timed out

Never expected to be in a situation where I could not shut down a machine cleanly like that. But maybe I was missing something?