I went in for an interview once, where they told me about the product, explained how they use MongoDB for their database, and then explained how building out all the relational DB commands on top of mongo was a total bitch. Then asked me to whiteboard how I would write the JOIN function on top of Mongo, which is what they had to do. I answered their question, but stated my opinions on mongo and asked why they even bothered to use it, because their product aligned so much more with a relational ACID database. The engineering lead guy went red in the face and we debated the decision. Did not get the job.