Earlier quoted context omitted.
Any data can be modelled in a relational way, I guess. It doesn't mean it's the best representation. Try modelling a cyclic graph in a relational way and you'll quickly tie yourself in knots trying to update and query it. The point is, relational databases a great for storing data that you've decided to model relationally. If you decide not to, then you probably want some other sort of database.
The point is, relational databases a great for storing data that you've decided to model relationally. If you decide not to, then you probably want some other sort of database. Sure. But that database is not MongoDB.
The "you only need relational databases" mantra bugs me though, because it's so obviously not true.