I have seen a few rather large, production mongodb deployments. I don't understand how so many people chose it as their basis of their applications. There are a not-negligible amount of mongodb deployments I have seen that basically treat mongodb as a memory dump, where they then scan from some key and hope for the best. I have never seen a mongodb solution where I thought that it was better than if they just chose any sql server.

SQL or rather just some schema based database has a ton of advantages. Besides speed, there is a huge benefit for developers to be able to look at a schema and see how the relationships in the data work. Mongodb usually involves looking at a de facto schema, but with fewer guarantees on types relations or existence, then trawling code for how its used.