Mongo DB is a NoSQL database and not an RDBMS. It doesn't provide the same level ACID compliance. The Pega platform has been built to install and work with a RDBMS. There are scenarios where the product interfaces with NoSQL technologies and we are actively looking at improving that with future releases of the product.
That said, we would still like to understand why your management would like to use Pega installed on Mongo DB. Providing more detailed use cases would help us.