I don't develop in JS so can't comment on DX there, but I've found the DX to be pretty good when using protobuf in other languages. That's mostly been down to

I don't develop in JS so can't comment on DX there, but I've found the DX to be pretty good when using protobuf in other languages. That's mostly been down to having IDE autocompletion for data structures and fields once the protobuf code's been generated. For many JSON APIs I've worked with there's only been human readable documentation, making them more error prone to work with (e.g. having to either craft JSON manually for requests, or writing a client library if one doesn't already exist).