Currently request and response messages are serialized with JSON, so only types supposed by JSON can be passed between a worker pool and its workers. This prevents, for example, a Date instance from being returned by a worker function.
Look into using BSON (or another faster/more capable serializer?) so more built-in types can be deserialized unmolested.
When child_process.fork() "advanced" serialization is used, errors occur. Look into why?