I have used Mongo to store medical data that had to be stored for 7 years. We set it to make sure that all three servers in the cluster did a write acknowledgement.
Thank you! As an aside, is there a reason you're not using a write concern of "majority"? If you have w:3 in a three node cluster then if one node goes down, writes are going to start throwing wtimeout errors (assuming you have wtimeout set) even though the data may safely be written to a majority of nodes. We generally recommend setting w:"majority" for this reason.
It was recommended by our vendor. I asked why, he was just paranoid. But, most of the data was entered on a semi connected mobile device, stored on the handheld and synced when they had service. If it wasn’t immediately writable to the server, it wasn’t a show stopper.