Ask HN: Best platforms for conversational UX development

I've recently started looking into building conversational UXs. I've looked at Amazon Alexa and also briefly looked at Actions on Google & API.ai.

Does anyone here have comments on where each of them shine, and what their shortcomings are? I'm particularly intrigued by how you can build conversational agents for multiple platforms on API.ai, but their documentation seems a little lacking at the moment.

Are there other platforms that are worth considering at this time? I'd imagine Apple will likely open up SiriKit to arbitrary intents soon.