With REST, for example, you usually have a smaller set of well defined APIs whose surface area is pretty visible and it could be custom optimized up-front or even disallow certain kinds of queries. GraphQL seems to provide enormous flexibility for the front-end engineer to generate any kind of request that it might not be possible upfront to anticipate all the kinds of requests that will be made and optimize them?
While it might be orthogonal to the design decision, it might add to the amount of unanticipated work that will be required just because of the enormous flexibility.