Former company rented space at an enormous data center. Place had bulletproof power in place - multiple sources, plenty of UPS, generated power for days. No worries, right?
Not so much. We got hit twice about a month apart, two power "hiccups". Downtime length doesn't really matter, if the power goes out for 5 seconds or 5 days, you've lost everything in-flight, computers are going to reboot, etc. We lost a fair number of harddrives when all of the servers cycled. Major PITA.
Much, much later I heard (unofficially, through an engineer there) that the outages were caused by a disgruntled employee who knew where the weak points were.
So - it happens. Not unimaginable.
Then their backup power plan was junk. In my experience power fail or fluctuation doesnt cause computing outages. Sounds like you and they cheaped out on that plan or didnt understand how it should go together.