There have been many posts about NUCs failing to wake up and/or getting completely bricked after the Linux kernel puts the device to sleep. It would seem there is an incompatibility between how the Linux kernel puts the NUC to sleep, and the NUC's hardware or BIOS. I don't have a clue whose fault this is, but it SEEMS LIKE* a successful workaround is to force the Linux kernel to use synchronous power management, meaning the NUC's hardware devices will be shut down one by one, rather than all at once in parallel (which is the default).
* I have not reproduced the problem after implementing this solution, but that doesn't mean it's solved -- it could just be a rare issue, or I might just have been lucky since following these steps. Please reply with your experiences so we know if this info is correct.
If your NUC is bricked and will not power on, you can probably recover it by disconnecting the CMOS battery for a short time. Briefly: Disconnect power. Flip over. Remove four foot screws. Ground yourself. Expose sensitive underbelly. Remove M.2 drive if needed (it may block access to one of the motherboard screws). Remove two black screws holding motherboard down. Gently lift motherboard out, being careful not to scratch its components or circuit traces against anything hard or metal. Unplug CMOS battery for a minute, then plug back in. Reverse steps. NUC should now turn on when power button is pressed.
TO PREVENT THIS PROBLEM FROM HAPPENING IN THE FUTURE:
sudo bash
cat << EOF > /etc/systemd/system/disable-async-power-management.service
[Unit]
Description=Disable asynchronous power management
DefaultDependencies=no
OnFailure=emergency.target
[Service]
Type=oneshot
ExecStart=/bin/sh -c "echo 0 > /sys/power/pm_async"
RemainAfterExit=yes
[Install]
RequiredBy=sysinit.target
EOF
systemctl start disable-async-power-management.service
# The next command should print 0 and not 1
cat /sys/power/pm_async
systemctl enable disable-async-power-management.service