"Did they test against a single MongoDB server or cluster?"

Couchbase: 9 nodes on 9 AWS instances (1 node per instance). MongoDB: 18 nodes on 9 AWS instances (1 primary and 1 secondary per instance).

"Well if you pick a good shard key it should be able to go to a single node."

Not necessarily. From the docs: In some cases, when the shard key or a prefix of the shard key is a part of the query, the mongos can route the query to a subset of the shards. Otherwise, the mongos must direct the query to all shards that hold documents for that collection.

If you query your data in many ways, you have lots of queries not based on the shard key. Considers users. What do you shard on, email? What if you want to query by location, age, status, and/or preference?