Compared to MongoDB it offers predictable performance, infinite scalability, outstanding stability, and tiny cost.
When using MongoDB at scale, you encounter all sorts of issues that are not advertised in MongoDB promotional materials. You get limited by the number of connections, not compatible drivers, and unexpected bugs in new releases.
With MongoDB, it's easy to get started quickly because it doesn't require you to think about data structure in the beginning. When you hit a certain scale you either spend months reengineering your DB or paying loads of cash to scale your cluster.
On the other hand, DynamoDB is much limited in its querying API, but that's a good thing. It's very hard to build a not scalable or low-performance database with DynamoDB. Also, you get all the perks of native integration into other AWS services.