DocumentDB is a managed database service that offers a subset of the MongoDB API, with limitations in terms of compatibility, scalability, and features compared to MongoDB Atlas. DocumentDB's distributed architecture is narrower and less scalable than Atlas, and it lacks advanced features like sharding, role-based access control, and integration with real-time event facilities. The service also has compatibility issues with local development and can be challenging to develop against. In performance tests, DocumentDB was found to be inferior to Atlas in many cases, particularly under write-heavy loads and complex query operations.