[stickfigure]

I feel like this should be pretty obvious. I'm pretty sure there are students in a bootcamp somewhere learning "joins make it easy to construct complex queries; denormalization eliminates expensive joins but sacrifices flexibility and adds potential data inconsistency".

Real world example: Consider an Order table and a Visit table; conversion rates aggregate orders over visits. In Mongo you can denormalize some of the Visit data into Order, but what happens when you change the logic for computing conversion ratios? Or you want conversion ratios broken down by web browser, source tag, or any of the other data elements that live in Visit but you didn't denormalize ahead of time?