Can you offer an example of the creators claiming that it does something that it really cannot do?
Also, while MongoDB does have its faults and some sharp edges, it is far from poor. For the most part, 2.x MongoDB is solid, performs well and generally without issue.
They claim the oplog ensures durability, but it actually gets written and fsynced quite late in the process. This becomes quite obvious when the master of your replicaset changes: clients will receive errors and you will get inconsistent data you have to clean up later. This is the sort of thing Postgres and even Redis get right.
They claim sharding works up to a high number of nodes, but even when following their recommendations, it's pretty easy to get inconsistencies. Also, many useful commands don't work (or work differently) with sharding. This is the sort of thing Cassandra and HBase get right.
They used to claim you can have control over whether reads/writes happen on the master or slaves, but not all commands supported this differentiation. There's was safe and masterOnly, but they don't always apply (and amazingly enough, don't always work). This is the sort of thing Cassandra gets right, and at least newer Mongo (and clients) support more granularity, and more consistently.
They claim certain benchmark numbers, but neglect to mention they're done without durability or any router/replica/shard setup.
It's nothing overly enthusiastic creators of a new database wouldn't be expected to do, but still annoying.
Also, while MongoDB does have its faults and some sharp edges, it is far from poor. For the most part, 2.x MongoDB is solid, performs well and generally without issue.