Although I am a heavy user/believer of MongoDB, one caveat that is overlooked in this article is the administration part of Mongo Clusters.

Even the simplest replication needs 3 servers, add sharding to the dance for extra performance and the server counter jumps up. For startups this is a major decision to consider as a full-time ops guy isn't always affordable. Luckily, PaaS services as MongoLabs and MongoHQ save the day.