Yet out of those, which actually implement the full spec (server-to-server, not to mention client-to-server, though I do think I remember reading that Pleroma has it), or even if they implement parts of it, do it in a compliant way? I would imagine the number is small because of the sheer complexity of the spec, and the many ways the same data can appear in different requests. I remember reading one of the Pleroma authors saying that they don't support JSON-LD in their implementation because no library for it existed in Elixir.

Like I said, I'm a fan of ActivityPub and its concept, but I think it's fair to say that spec is more complex than what it needs to be, and would have been better off strictly defining the shape of data it can handle.