Previously ubiquity did place itself under inhibitors to prevent unwanted operations happening in conflict with teh installer processes, or racing them.

There are some gsettings that ubuntu-desktop-installer does, but things need to be checked if they are in effect.

To check or enable:

• systemd-inhibit => probably all operations of shutdown, sleep, idle, handle-power-key, handle-suspend-key, handle-hibernate-key, handle-lid-switch => doing any of those things during installer running is dangerous.
• /usr/lib/udisks2/udisks2-inhibit => prevent processing disk drive events during installation (in the past this caused issues)
• check that lockscreen is inhibited (gnome-shell / gdm lock)
• check that display brightness settings are inhibited (do not dim screen / disable screen)
• check that OOM score is adjusted to prevent killing subiquity/ubuntu-desktop-installer

Some reference code may be found in ubiquity, i.e. ubiquity-wrapper and other places in the codebase.