I'd say Dynamo is extremely good at reading high volume, with the appropriate access pattern. It's very efficient at retrieving huge amounts of well partitioned data using the data's keys, but scanning isn't so efficient.
You can only ever fetch 1MB of data at a time though, even when using the more efficient query method (as opposed to scan). If your individual entities are not very tiny, it is hard to get for instance 2M items back in a reasonable amount of time.
