I have not yet used all three, and doubt there are very many people who have. Dialog system APIs are still very new. I didn’t even knowhad an API until I read this question and went and looked. I can make a few general observations to start with.
First, dialog system APIs are distinct from NLP APIs, in that dialog system APIs interpret natural language queries and return natural language replies, generally in textual form. Whereas, NLP APIs usually perform one or more. (I don’t know of any good dialog system constructed solely on top of either NLP APIs or “tasks of natural language processing”.)
As with any API, there are different levels/kinds of authentication to consider, from no security to simple single token authentication, to complex handshake authentication (OAuth). Also, there are various kinds of knowledgebase control, from no control over the knowledgebase to partial control, to complete control. Especially with the new breed of hybrid machine learning/neural network dialog systems, as a business user I am very unsatisfied with unpredictable replies, from unknown sources, and also unsatisfied not being able to repatriate 100% of my modified or learned data in the knowledgebase.
See also my Quora question: