[merlynn]

In the spirit of complete transparency, I work at MongoDB. Depending on users' specific needs and the features of MongoDB they use, the differences could go quite a bit further than 'just' support. Some of the important differences to keep in mind include regional availability and approaches to resiliency (Atlas allows you to scale beyond a single node and offers multi-region clusters; DocumentDB limits you to a single primary), role-based access control (DocumentDB currently only supports user that has full access to all databases in a cluster; MongoDB allows fine-grained control), freedom from vendor lock-in (With MongoDB you can always choose to host your own clusters whereas DocumentDB does not afford you this option). These are only a few of the differences between the hosting platforms, on top of that, you would, of course, be limited to an incomplete set of features (https://docs.aws.amazon.com/documentdb/latest/developerguide...) compared to MongoDB 3.6 and you would be missing out on improvements available in MongoDB 4.0 and future versions.