Is there any kind of downside to using react apollo to call queries/mutations, with aws appsync?
The Apollo client should work just fine with AppSync for queries/mutations.
One potential downside would be missing any future potential AppSync-specific optimizations or features that might be released the the AWS Mobile SDK team, as well as future ability to integrate subscriptions, which would require the AWS-built SDKs, or modifying the Apollo SDK, since the Apollo SDK would not work with AppSync subscriptions out-of-the-box.