The zOS boxes we run are super-robust.
They've been running for over 20 years with an average of one hardware issue that affects applications every 3 years, and I can count on one hand the number of times any of those boxes have been rebooted (IPLed in mainframe lingo) during that time period. They can be upgraded while still running. The memory on the boxes can be partitioned to isolate applications from each other.
http://www-03.ibm.com/systems/z/hardware/