Some of it is also that, much like in comparison to schema-less protocols, it's a bit of an apples-to-oranges comparision at the RPC level, since capnproto's RPC is so much more expressive -- but also complex to implement. I think only some of the difference in available implementations is due to reduced engineering effort; the long-tail of serialization-only implementations is testament to the fact that implementing cap'n proto rpc is Not Trivial. Unfortunately, I think a lot of this complexity is inherent to what cap'n proto rpc is trying to do.