Atlas makes sharding easier, it's just one button where you define your partition key.
I honestly believe that this is a key differentiating feature for many databases. ElasticSearch, ScyllaDB, others too work the same way: Every node is equal and in order to scale you just keep adding more boxes, end of story. Compare that with what you have to do with MongoDB.
DocumentDB is a great DB, but in the end of the day it's a forked version of 3.6-ish so it's missing a ton of new features like multi-collection ACID guaratneees. Plus, whenever you use DocumentDB you're accessing it via the MongoDB official drivers, so you'll be handling that compatibility mess yourself.