Based on how the query executor currently works, I wouldn't call it scalable. Well sure, it can scale, but that implies that it's more efficient when it isn't in many cases. Basically the more complex your GraphQL query gets the more steps the GraphQL server has to take to resolve it, and figuring out how to combine those operations into something resembling a single SQL query isn't an easy thing to do. It scales so well for Facebook because their implementation only reads from a cache so they don't really have to worry about it.

Aggregating various data points manually in a predefined way may be less flexible than GraphQL, but it's much easier to optimize when things get complex imo