The problem with the way JXcore did the SpiderMonkey port is their extensive use of C++ macros - not unlike NAN for Node.js. This makes the code hard to debug and maintain. The Microsoft Chakra Node port is more elegant because they've mimicked the V8 C++ API making it much more likely that it will be merged into Node.js and IO.js. In time I suspect Mozilla and other javascript engines will make V8-compatible API shims similar to what Microsoft did:

https://github.com/Microsoft/node/tree/ch0.12.2/deps/chakras...