If new fields show up in a json response just ignore them. Why would you need to change the client if new fields show up in the response?

depends. i'd be writing the client such that it just lists all the fields, and then add special handling for the fields that need it. when the backend adds new fields, they will just show up and i just need to fix the formatting. with graphql i'd have to ask for those new fields, and thus make changes in two places. and in addition the backend team has to tell the frontend team about the new fields (instead of letting the api speak for itself), making it easier to accidentally skip a field.

One problem is performance.

(Most) GraphQL clients are optimized for relatively small/simple objects being returned, and you typically pay a cost for every single edge (not node) returned in a response / cached in memory.

It can quickly get to the point where your project is spending more time per frame processing GraphQL responses & looking data up from the cache than you spend rendering your UI

it sounds like they are just dumping every field to somehow show the user without any validation/specific rendering logic for each field