We didn't have downtime for various reasons but the ELBs we were using failed and the queue of starting instances was too big to see our few ones restarting.
The main systematic issue in EC2 is EBS, take that away and it will almost completely remove downtimes.
The problem with ELBs is that they are themselves EC2 instances and use many of the global services for detecting load, scaling up, etc.. Like all of AWS's value-added services, they are therefore more likely to fail during an outage event, not less likely, as they depend on more services.
The main systematic issue in EC2 is EBS, take that away and it will almost completely remove downtimes.