It's fine if most implementations of RPC are Level 1, even the main C++ bindings; due to the design of the protocol there only needs to exist a single Level 4 implementation written in any language, and all other implementations can then piggyback on it for their own needs beyond Level 1 (so cap proxying, etc. all come "for free" once someone does that.) I've mentioned Level 4 support to Kenton a while back and the basic answer was "I haven't needed it so it's not done." It's not intended for there to be a Level 4 implementation in every language or anything.
The biggest issue is undeniably that the language maturity isn't quite as solid as the alterntaives. You've got C++, Rust, Python, C#, maybe one or two others. Only C++ is first class. So gRPC and Protobuf win here. But the actual RPC design and IDL design of Cap'n is solid and way more well engineered than the alternatives, IMO.
I have no opinions on the ZeroVer thing. Cap'n Proto is very stable in my experience but I see why others would raise their eyebrows.
A lot of their tests were checking for Exception raises and were basically testing AttributeError due to a change in interface instead of the initial error they were testing for.
The biggest issue is undeniably that the language maturity isn't quite as solid as the alterntaives. You've got C++, Rust, Python, C#, maybe one or two others. Only C++ is first class. So gRPC and Protobuf win here. But the actual RPC design and IDL design of Cap'n is solid and way more well engineered than the alternatives, IMO.
I have no opinions on the ZeroVer thing. Cap'n Proto is very stable in my experience but I see why others would raise their eyebrows.