MessagePack/msgpack is great for that middle ground where JSON ser/des is too slow, but you don't have enough engineers to justify the maintenance burden of a heavier-weight schemaful protocol.

I did end up writing a simple schema verifier for Ruby (ClassyHash, on GitHub) in one of the jobs where I used msgpack, but I no longer have access to maintain it. My benchmarks showed msgpack+classyhash was faster than native JSON (didn't test oj I think) and other serialization formats, and faster than all the other popular Ruby schema validators at the time.

Tldr: msgpack rocks, use it instead of JSON for internal services