The following is from the AWS web site [1]:
> Availability Zones are distinct locations that are engineered to be insulated from failures in other Availability Zones and provide inexpensive, low latency network connectivity to other Availability Zones in the same Region. By launching instances in separate Availability Zones, you can protect your applications from failure of a single location.
No mention of tradeoffs.
The quoted statement doesn't say that isolation is 100% or that multiple AZs can't ever ever fail at the same time. It says that if only one AZ goes down and you have servers in another, then those servers will still be up, which should be obvious. Insulated doesn't even mean the same thing as isolated.
Even the name, 'Availability Zone' implies that it is isolated from other 'Availability Zones' in the same region. And that text I quoted does nothing but substantiate that inference.
I just think that Amazon are misleading here. Maybe they shouldn't call it an Availability Zone.