I had one CyberPower unit that had like a ground fault (the ground light on the back would flicker), so I opened up the case and found the lose screw. Tightened it up and it worked once again
Generally though I don't think I've had any full unit failures, always just the battery. Though over the years I got rid of the non-PFC/sinewave ones since they'd make the computer PSU whine pretty bad while on battery.
As for testing it, I think it's better to leave it plugged into the wall but flip off the breaker at your electrical panel. If you unplug it from the wall, then you are removing the ground.
Generally though I don't think I've had any full unit failures, always just the battery. Though over the years I got rid of the non-PFC/sinewave ones since they'd make the computer PSU whine pretty bad while on battery.
As for testing it, I think it's better to leave it plugged into the wall but flip off the breaker at your electrical panel. If you unplug it from the wall, then you are removing the ground.