Something I haven seen here:
If the attacker gains sufficient permission to install even an official UEFI firmware, correctly signed by the system manufacturer, they can still potentially leave the computer in an un-bootable state by forcefully powering off the computer at an opportune time during the process.
The update code in modern firmwares usually tries to minimize the amount of time the computer spends in a state where a power failure will cause corruption of the firmware, and some firmwares even have a recovery mode which will activate in such a case.
However, many of these systems aren't completely bulletproof. Although they offer good protection against random power failures, a well-timed poweroff could still knock it dead if the firmware doesn't have a robust automatic recovery feature.
Also, one may not even need to attack the main system firmware. Pretty much every device in a modern PC has a firmware of some kind, and many of them can be updated via software. These devices are also often less secure. They may accept unsigned firmwares entirely, or at least be less resilient against malicious poweroffs during the update process.
If you destroy the firmware on the power controller, storage controller, storage device, video device, or input controller, the system is may become just as unusable as if you had attacked the UEFI.