Zsh autocompletions for most systemd things work, but the command which I use most often is systemctl suspend and here autocompletion does not work.
Arch Linux package version 5.0.0-2
Steps to reproduce:
Start zsh and type "systemctl sus" and press <tab>. It should be autocompleted to "systemctl suspend", but nothing happens.
Just add the line "suspend:Suspend the system" to /usr/share/zsh/functions/Completion/Unix/_systemd inside _systemctl_cmds=(..), probably on line 87.