"Oversold" is the key word here. And that's a marketing concept. To dig deeper, the problem has and always will be that MongoDB toes the line between a caching product and a database product, yet 10gen markets it simply as a database product.
MongoDB is flexible and fast, but practically speaking it's a hybrid product that defies simple categorization. That means it must be well understood to leverage it. Ultimately that may also be it's Achilles heel, but in the short term, it's extremely frustrating that 10gen can't embrace this fact. Instead they perpetuate marketing that causes the product to be perceived as flawed by their target audience.
Once you've discovered this, you can use it appropriately (either by overriding the default silent failure to use it as for durable persistence, or only using it for caching or as an eventually consistent store) to reap it's flexibility and speed.