Amazon do not say that DocumentDB is a drop-in replacement, neither do I. Moreover, I have outlined specific incompatibilites between Mongo and DocumentDB we have encountered for our use cases; RBAC controlled document filtering is not available in DocumentDB, either. Therefore, it won't suit everybody.

However, for simple to medium complexity projects, especially for brand new ones, DocumentDB is a viable and a more affordable alternative to Atlas MongoDB with a decent level compatibility.