Lately i was able to catch some pretty strange output, see here
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[EHCI] Doorbell acknowledgment timeout (intr=0x00000003)!
[EHCI] Doorbell acknowledgment timeout (intr=0x00000003)!
[EHCI] Abort didn't succeed!
Not sure if it's a (USB) hardware problem with one of the attached devices (though i only have a perixx mouse/keyboard combo and a midi to usb converter plugged in, since years) or a usb stack/EHCI bug.
Anyway, when this error doesn't come up chances are high the key combo for cold reboot works fine (but still not 100%)...