gRPC is very opinionated. Protobuf allows some things that tend to be design mistakes (e.g. required fields). A lot of this has been corrected with proto3, with subsequent field behavior annotations described here: https://google.aip.dev/203

The whole of aip.dev is a collection of API design principles published by Google that are externally relevant. Many of the proposals pertain specifically to gRPC-style APIs.

I've personally found gRPC APIs to be pleasant relative to many HTTP APIs since the use of an IDL can be helpful when interacting with such APIs. I don't have enough GraphQL experience to make any conclusions on that comparison. But the sense that I get is that GraphQL may be more appropriate once you reach a certain scale but is probably overused when a simpler technology would suffice.

Disclaimer: I work for Google but the views I express are my own and not necessarily those of my employer.