People. In my experience faulty generators come from bad maintenance. We have one in every site and 2 generators in locations where there is no electricity (mostly Caterpillar). They mostly fail due to lack of/bad maintenance.

Like others said, yes they are big and with lots of moving parts. But if you take care of them (every month by the book) no one should have any unexpected problems.

For me is mind blowing that OVH did the last power failure test in MAY. WHY so rare? IMO that is 1st mistake.