I think the author summed it up well: "There is a niche where MongoDB can work well."

s/MongoDB/Any technology/g

The size of the niche varies. The lesson is to be sure the choices you make are appropriate for your situation, and be aware that things may change if new requirements emerge or scale needs to go beyond what you projected. These concerns are not specific to MongoDB. It is a rare project that goes from prototype to small scale to large scale on its original implementation technology choices.