Having said that, I very biasedly say that there's a much better solution to this connector, which doesn't flatten out the MongoDB data: it's called ToroDB (https://github.com/torodb/torodb).
ToroDB, open source, speaks the MongoDB protocol, transforms documents to relational tables (without any kind of flattening, and without having to define any schema) and stores data in a RDBMS. More precisely, PostgreSQL.
Current development version (repl branch) speaks the replication protocol, and hence can replicate live from a MongoDB into PostgreSQL. No connector needed, no flattening, no FDWs, nothing else. Just add a new "slave" (ToroDB) to your replica set and you're good to go.
It goes even further: if you want pure data warehousing, ToroDB will soon support GreenPlum. Some initial benchmarks (http://www.slideshare.net/8kdata/torodb-scaling-postgresql-l..., slide #42) show 25x-75x improvement between doing aggregate queries in MongoDB and their equivalent queries in GreenPlum's distributed SQL.
Now that MongoDB 3.2 ships with PostgreSQL "included", feel free to try ToroDB. It's always better the original :)
Note: I am a ToroDB developer.