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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

Edit: http://www.youtube.com/watch?v=b2F-DItXtZs

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.