You may have heard that MongoDB has issues with scaling out. You may have heard that Viber is replacing MongoDB with Couchbase Server. Have you heard that MongoHQ has to scale up?
I agree with MongoHQ on one thing. It's easier to scale up. While scaling up may be the only solution for MongoDB, it is not the right solution. What happens when there is no more room to scale up? I think that MongoHQ would prefer to scale out, but they realized that MongoDB was not engineered for it.