Our company has a mobile app that allows customers to make payments, update contact info, look at purchase history, etc. We have an enterprise data warehouse that works great for reporting, but it can't handle the load for 100,000 queries per day - at least not while providing consistent sub-second response time that a mobile app requires. Our solution is a micro-services style architecture with a customer centric document database in Azure CosmosDB. Each JSON document contains all the history for a single customer, so it's like one read or maybe a handful of reads. Azure's feature to throttle RUs (request units per second) up or down as needed is awesome - and that's something a non-hosted MongoDB database can't do. If all you're doing in CosmosDB (or MongoDB) is bookmark lookups on something like CustomerID, then encryption / decryption can be performed in the middle tier - hiding the contents of the document from the prying eyes of untrusted DBAs.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho