DocumentDB is a classical example of a service whose quality is directly affected by licencing issues.
In the world of cloud, many services are managed versions of open-sourced software. However, sometimes, things get trickier due to licencing. Cloud vendors might be wishing to host an open-sourced service, but they could be legally prevented from utilizing the generally available code in their SaaS offering. As a result, they end up writing a proprietary counterpart with a compatible API. Since this is simply playing catch-up, it is likely that the code owner’s hosted offering (if it exists, of course) is, by default, in an advantageous position.
A great example of such a relationship are AWS DocumentDB and MongoDB Atlas. Both platforms are offering managed versions of MongoDB API-compatible NoSQL databases, but MongoDB Atlas seems to be more complete - possibly due to the unparalleled understanding and control over the MongoDB codebase.
DocumentDB is boasting a compatibility with the MongoDB API, but according to MongoDB themselves, this integration is far from exhaustive. Even a quick glance at the documentation leaves the impression of spottiness. On top of that, AWS counterpart is 2 major versions behind (5.x vs 7.x). What’s more, some of the features that are yet to be replicated in DocumentDB revolve around tiering out data to S3 - which, being another AWS service, should have been high on their priority list.
For the exact case of MongoDB, we usually recommend MongoDB Atlas Database over the DocumentDB. It can be hosted in AWS VPC (in the same region as your services), so together with VPC Peering this ensures a seamless integration with the remainder of the infrastructure in AWS. The only justifiable use-case for DocumentDB are very strict compliance requirements. However, MongoDB Atlas have put a significant amount of effort into this aspect of their offerings as well.
We’d like to underline how important it is to thoroughly research the competition before integrating a service. The largest cloud vendors are never clumsy, but sometimes there are other reasons (like licencing), which put their competitors at a significant advantage.