The problem with ElastiCache -- and why I rejected it as an option -- as that they make you define a 30-minute "maintenance window" where AWS can apply patches and reboot your cache instances. In practice I've heard that this happens rarely, and when it does, the downtime is short, but it can in theory cause longer outages.

And in the case of both the redis and memcached backends, if the maintenance requires restarting redis/memcached or rebooting the instance, you lose all data in the cache (at least up until your last backup). For this particular project, that amount of downtime would easily cause a real outage for customers, and was unacceptable.