As much as I love MongoDB it has way too many issues to use it as a primary data store for financial transactions. I hope they backed up and tested their backup recovery. Something tells me they're dealing with a data corruption/loss which wiped out their master and slaves without a backup. Perhaps if they've got decent logging they can piece it together with logs.
I barely trust MongoDB with my personal projects. It's shot me in the foot enough given they're not a very stable vendor (case in point, see the 2.4.0 replication bug -- we didn't get hit by that thankfully).
We've had issues where databases can get on divergent paths, then MongoDB will keep up to 300mb of the stuff it can't match in a directory and after that you're hosed.
