JSON API seems overly verbose to me. It feels like like a direct descendant of the SOAP / WSDL era -- or maybe I'm just scarred from having written and consumed SOAP webservices.
Have you actually implemented something following this spec? My experience is that it's somewhat okay-defined for consuming data, but the spec itself is wildly incomplete in the other direction. Looking at the repo and discussion forum, the maintainers are inactive, with tons of open questions unanswered.
I implemented two APIs with it, mostly read oriented to be honest. The biggest problem we faced is the lack of maturity of Android and iOS SDKs for it.