We like to embrace different technologies and use each for its strength. MongoDB is excellent for a whole class of use cases and we like it a lot, but we also use other persistence solutions as well. Check out Polyglot persistence post by Martin Fowler, it's an awesome read. http://martinfowler.com/bliki/PolyglotPersistence.html
MongoDB is web-scale
Stripe uses mongodb, if its good enough for them, its probably fine for financial info. As I recall, there is some aspect of mongodb that lets it scale up without maintainance, making it good for a service that just can't go down.
Is mongodb really a good solution for storing things? IIRC one concern about coinbase (YC) is that they used mongodb for storing financial information.