That I understand.
The problem is that on the way you basically reduce your server to the thin layer over DB. Exposing querying API means that you significantly lose control over what, where and how you provide from the server.
We are currently considering using GraphQL in current project and I took a long look at both the standard and library (Absinthe for Elixir). It looks terrible to me. Basically you need to provide yet another layer, supporting mostly generic operations. If your action is more then CRUD, you still need a REST API.
Moreover, you slowly lose control on what can be changed and where, your frontend basically eating logic, but unlikely to be working on it's own.
I understand that it helps when you have records with multiple associations and you want to increase performance and not load everything everywhere, but the cost appears to be huge in terms of maintenance. I totally get why FB is doing it, but it seriously worries me that they market it as an successor to REST, instead of an alternative with such-and-such tradeoffs.