We had a lot of operational experience with Mongo from our previous jobs at Atlassian, and started out storing almost everything in Mongo. As we scaled out, we migrated all of our high volume data into other stores-- analytics into DynamoDB, searchable data into ElasticSearch, etc.
The last remaining piece is our core data, and there's no strong push to move it out of Mongo.
The last remaining piece is our core data, and there's no strong push to move it out of Mongo.