Renato Alves explains why GraphQL and WordPress work well together:
“…to author a GraphQL API you only need a one-time effort…. and another advantage of using GraphQL is its ease of integration with multiple data sources.”
There are some downsides, however. There are some “rough-edges” (like caching), access control, and a “steep learning curve.” Migrating completely to GraphQL without understanding its risks would be a bad mistake.