Overall, my experience of OS/390 is that it is a rock solid basis for a system. I remember a power outage that took out all the DASD. OS/390 sat there bleeping that it couldn't access disks, but otherwise stayed up and restarted itself once the DASD were IML'd back online.
I'm reminded of a situation where some cable-laying monkey had removed an entire single row of floor tiles behind one of those washing machine-like disk drives. So, after a vigorous bit of seeking, the legs underneath the drive inevitably started diverging from the vertical, and the drive decided to take a plunge into the chasm.
After undoing this mishap (I'm unaware if the cable monkey was made part of the solution) and spinning the drive back up, VMS merely reported "Drive improperly dismounted".