DynamoDB has had major improvements in the last few months: e.g. you get dynamic capacity provisioned tables (avoids re/write capacity exceeded exceptions because of capacity planning uncertainty), and transactions, to name two. However, even if you have a hosted RDBMS it has an implicit read and write capacity throughput that you need to design for (e.. hotspots in partitions), you just hit it a bit later in your project. The bounded latency at scale (throughput, and size of tables) is the main win for DynamoDB.