And yet somehow they seem to have gotten it to work. I run a few Zookeeper-based systems where ZK just seems to work and I don't have to look at logging statements, either. I've never dealt with Zookeeper downtime that wasn't Amazon's fault.
No, individual nodes have gone down based on hardware problems. The system stays up. Jespen has given Zookeeper probably the most ringing endorsement of anything it's tested, so I don't know what you're on about.