That is orthogonal concern to what is being discussed if you run geo distributed Mongo cluster you will either have slow queries or will compromise on data integrity.
It's not orthogonal because it is harder to have master-master cross data center setups in relational databases even in scenarios where eventual consistency is acceptable (such as a use case for a piece of our infrastructure we have).
Sure MongoDB may not be the best fit for it but my point is more that these are scenarios where horizontal scaling is an important consideration makes more sense for some nosql solutions than for sql solutions. It's not just about single box performance.
