What about scalability? Trying to cluster and shard MySQL is a very difficult task, but with MongoDB it is trivial. No schema can be good, but scaling out easily is the big plus I see.
We did not have this experience when I worked at a large datamining company. It was a nightmare.
Mongo markets ease of sharding as an advantage, and if that is not the case at times, it limits the attractiveness of losing RDBMS features.
It is just a setting you know that right ?