This post will be short...
I'm not a big fan of systemd and honestly I can't understand how this disease (which became an epidemy) could have happened to Linux. Sometimes I wish an eternal insomnia to person who is responsible for this (I am joking of course). Anyway systemd is (unfortunately) part of my OS and (despite numerous benefits) it often offers me surprising moments.

[root@ol7-01 ~]# reboot
Failed to start Connection timed out
See system logs and 'systemctl status' for details.
Failed to open /dev/initctl: No such device or address
Failed to talk to init daemon.

Of course the same result was produced by systemctl reboot, init 6, reboot -f -r now or even systemctl -f reboot

Last resort has been in using the deadly force in order to reboot the system:

[root@ol7-01 ~]# systemctl -ff reboot

I used the "deadly force" term for "-ff" (you can use --force --force, or -f -f) because it's appropriate for logic behind this directive. Take a look what man page for systemd says:

"When used with halt, poweroff, reboot or kexec, execute the selected operation without shutting down all units. However, all processes will be killed forcibly and all file systems are unmounted or remounted read-only. This is hence a drastic but relatively safe option to request an immediate reboot. If --force is specified twice for these operations, they will be executed immediately without terminating any processes or unmounting any file systems. Warning: specifying --force twice with any of these operations might result in data loss."

So use this option as a last resort.