BIOS update bricked a board, but was it the update or the user?

So last Tuesday I flashed a fresh BIOS on a client's 2018 Z390 board and it went sideways. Post screen froze, no beeps, dead. I spent 3 hours with a clip programmer trying to recover it. Got it working again with a backup chip, but here's the thing: the client admitted they had a power strip with like 5 things plugged in and maybe a flicker happened during the flash. Now I'm stuck on the debate: do we blame the update itself or the lack of a UPS? I've seen 20+ BIOS flashes go smooth, but one brownout can nuke the whole thing. Has anyone else had a flash fail from a power dip vs a bad file? Or do you just roll dice and hope every time?