Back in the late 1990s I was involved in provisioning a large Sun e15k. Not indestructable but nearly.
It broke. You know what happened? The factory roof leaked and poured water onto the DC sub-building which the roof then collapsed onto the e15k which promptly blew up and caused a spectacular fire, halon dump and about a month of work arguing with insurance companies and guys with shovels.
In that circumstance, it doesn't matter what promises the vendor make. That's still all your eggs in one basket.
Buy two and keep one somewhere else didn't help either as the network termination, switching and routing layers were down and all the people using it were about 300 miles away from the backup location anyway. So some poor fucker had to dismantle the backup e15k and disk arrays, bring them in a large truck[1] to the original location and erect a temp DC in a portakabin outside the building.
Edit: We would have been better served with two smaller DCs with off the shelf kit on the same site but different buildings running a mirrored arrangement. All for pocket change compared to a zSeries...
That's what the company I work for now do. We have off the shelf kit,SAN replication, ESX, redundant routing and multiple peers in different locations.
[1] imagine the shit if that truck crashed.