I appreciate the vagueness of this question, but: I can’t help but notice that virtually all non-C++ versions of capnproto do not implement the RPC interface. Is this because the RPC interface is especially difficult, because it’s not widely used, because it’s not trivial to invoke via JNI/ctypes/etc., or what?
I think a lot of people are happy just using the serialization protocol, as it does stand on its own. Cap'n Proto’s object-capability layer is somewhat more complex than other RPC protocols, but that’s because it includes support for killer features like promise pipelining. The Python and Node implementations demonstrate that it’s not too hard to wrap the C++ implementation.
As sandstorm.io gets more popular, there should be a growing interest in RPC implementations in other languages.