EBS performance is notoriously variable by orders of magnitude. Amazon is working on this, however my personal belief is that using block storage creates an undesirable link between your storage and compute layers. Using S3, RDS or another NoSQL solution is a far more modular approach.

That being said, all apps are different and some may need block storage. I'm not saying it should not be used, however when making that decision the relation and hard dependencies created should be understood.