Mongo does not support joins between collections
Mongo does not support transactions across multiple documents, but there are ways around this by including all data inside the document which can be rolled back if there is a fail.
Mongo has a flexible data model which allows your database schema to evolve over time without migrations
Relational databases are best suited when applications require complex multi-row transactions e.g. a double-entry bookkeeping system. Mongo is not a drop-in replacement for legacy applications built around this model.
| MySql | MongoDB
------------- | ------------- | --------
Rich Data Model | No | Yes
Dynamic Schema | No | Yes
Typed Data | Yes | Yes
Data Locality | No | Yes
Field Updates | Yes | Yes
Easy for Programmers | No | Yes
Complex Transactions | Yes | No
Auditing | Yes | Yes
Auto-Sharding | No | Yes