If you don't have a primary key and foreign keys you don't really have a relational DB. And if you have a primary key you can't scale perfectly, because you ar

If you don't have a primary key and foreign keys you don't really have a relational DB. And if you have a primary key you can't scale perfectly, because you are promising global consistency. A non-zero price tag makes many applications impossible (uneconomic in practice). So that's a good reason to use a non-relational engine for them.