What a lot of people overlook is when Amazon takes something (DocumentDB with MongoDB compatibility) on internally is they are re-inventing the way the database works in such a way it's dubious if the community would benefit. I do think it would have been better to see this go the Postgres route where they could have continued to contribute to the core product but I believe MongoDB saw competition against their MongoDB Cloud product and decided to change the licensing. This neccessitated the need for a hard fork since they were not interested in working together. An AWS Q&A stated they are working on a completely independent code base that is API compatible with MongoDB 3.6, so there is nothing to contribute back to the MongoDB open-source code base.
DocumentDB much like Amazon Aurora has pivoted much of the database internals to depend on the AWS ecosystem. The whole concept of replica-sets on MongoDB are very rigid and inflexible compared to the elegance made possible by EBS and Amazon engineers.
MongoDB replication is measured in seconds meanwhile DocumentDB replication is "usually less than 100 milliseconds". This makes read replicas in DocumentDB more relevant for all kinds of real-time tasks vs MongoDB.
DocumentDB much like Amazon Aurora has pivoted much of the database internals to depend on the AWS ecosystem. The whole concept of replica-sets on MongoDB are very rigid and inflexible compared to the elegance made possible by EBS and Amazon engineers.
MongoDB replication is measured in seconds meanwhile DocumentDB replication is "usually less than 100 milliseconds". This makes read replicas in DocumentDB more relevant for all kinds of real-time tasks vs MongoDB.