I've done JSON-RPC at scale before and the one downside to it is that you have to write a custom caching proxy for readonly calls that understands your API.

With REST you can just use a normal HTTP caching proxy for all the GETs under certain paths, off the shelf.

Using a hybrid (JSON-RPC for writes and authenticated reads, REST for global reads) would have saved me a lot of time spent building and maintaining a JSON-RPC caching layer.

There is benefit to a GET/POST split, and JSON-RPC forces even simple unauthenticated reads into a POST.

The other issue with JSON-RPC is, well, json. It's not the worst, but it's also not the best. json has no great canonicalization so if you want to do signed requests or responses you're going to end up putting a string of json (inner) into an key's value at some point. Doing that in protobuf seems less gross to me.