Operational costs are in the eye of the beholder. We evaluated DynamoDB for a new use case but we're going with Cassandra because the storage costs alone more than made up for ops overhead. At least IMO, Cassandra's support for multicolumn range queries (via clustering columns) and the cheaper storage you can use obviate local secondary indices. (Cassandra has secondary indices as well but it seems like most folks prefer further denormalization--at least, that's what we're doing.)