Ubuntu Custom Partitioning: Failed to start D-Bus System Message Bus

As a courtesy to the next person who needs to figure this out: You have installed Ubuntu with custom partitioning and the boot process hangs, because systemd fails to start various daemons (systemd-resolved, systemd-oomd, systemd-logind, thermald, avahi-daemon, rsyslog), but most notably: Failed to start D-Bus System Message Bus.

To fix this, you need to rerun the installation with the boxes checked to have the installer reformat the partitions that you install to—even if they seemed to have empty file systems of the right type. If you try install onto an ext4 file system created by Gnome Disks (as opposed to one created by the installer), you are going to have a bad time. What a massive UX failure that the installer allows you to install in a way that results in a broken system.

You can use pre-created partitions, including LUKS containers, though.