Great question! My rule of thumb would be for server to server apis use protobufs. Also probably for Android/iOS/Flutter clients (depending on how well the protobuf compiler is integrated in your toolchain). For web clients, last time I checked, grpc-web was the most complete implementation/tool, but couldn't make it work with the rest of our stack. Maybe spending more time I could've, but it got really frustrating and I gave up.