It makes sense to use mongodb for commenting system where nobody cares if you lose a comment or certain comments are written before others.

But no competent architect would use mongodb for financial/currency transactions.

If coinbase did use mongodb, it will be inevitable they will have transactional issues. The more popular coinbase gets, the more transactions they process, the more issues they'll have. I don't envy the poor souls that have to track down these kinds of issues...