I think you just confirmed the OP's point -- MongoDB makes a good cache, not a good primary store. I'm guessing you didn't do updates into that MongoDB store, and always treated the SQL source as "authoritative" when it became necessary. Am I right?
The transition was difficult because of the sheer number of systems that imported/updated listing records in the database... there wasn't yet a 100% certainty that all records were tagged on update properly, so that they could be re-exported to mongo... each day, all records were tagged ... took about 24 minutes to replicated the active listings (about 50K records), and we're not talking "Big Data" here, but performance was much better doing search/display queries against MongoDB.