If I read it correctly, the server can fail but because the data is stored via iSCSI, it will not actually be lost (unless it is the appliance itself that fails)
the storage for the appliance can be outside the appliance (DAS or SAN storage). the appliance can die, but you just plug in a new appliance and point at the storage, and your back... as for the drobo comment, this should be faster than a drobo... and even ignoring the appliance fail, the off site backup is key.