MongoDB isn't the right tool here of course, but the rumoured attack could have been avoided using MongoDB's findAndModify command which gives you the atomicity you need to update a ledger.

The problem here is that developers not taking the time to understand their application and how their tools work. Yes, MongoDB gives you enough rope to hang yourself and I certainly wouldn't suggest using it for a financial ledger, but you could have a durable application based on it if you really wanted to.

However if developers of a financial system cant grasp the need for atomicity in the transaction path then they're going to be buggering things up all over the place.