>but I'm not sure how often reads/writes to mongodb just disappear out of thin air.

Which would imply you have to restore from backups continuesly. Or else any bug you run into might be the result of data corruption.

Its the fact that you cant even tell. And its not just about loosing data. If you rely upon a consistent world view (i.e. schedule payment, jobs scheduling, vms provision or shipping, account systems) the risk is unacceptable.