[ncmncm]

Probably the main difference from operating bolted to a test stand is vibration both from its own operation and from all the other engines operating at the same time, and instabilities in fuel flow caused by the same. The bottom end of a launching rocket is a really hostile environment. Just the noise would kill you. Top is marginally better, depending on what happens at the bottom.

It is hard to imagine how you would determine, after the fact, whether a rocket blew up because of material failure or something else, but they seem to do it, anyway during development when they have a zillion sensors attached logging everything in real time: "hmm, a millisecond before the explosion, this reading went out of tolerance, and then this one, then this one, and then the data ends."