Well at the bare minimum setting up proto files and knowing where they live across many projects.

If they live in their own project, making a single project be buildable with a git clone gets progressively more complex.

You now need sub modules to pull in your protobuf definitions.

You now also need the protobuf tool chain to be available in your environment you just cloned to. If that environment has the wrong version the build fails, it starts to get frustrating pretty fast.

Compare that to json, yes I don't get versioning and a bunch of other fancy features but... I get to finish my work, build and test pretty quickly.