It's possible to integrate QnA Maker to Power Virtual Agent but it looses one great functionality, multiturn / follow-up prompts.
It would be great to see better support for QnA Maker integration with the ability to make use of follow-up prompts.
My use case is a support-bot where I have more advanced "flows" as topics in PVA, but if no topic is found the QnA maker knowledgebase works as a fallback. My QnA knowledgebase holds a lot of questions and answers and in my opinion QnA Maker is a simpler tool to handle larger amounts of simple question-answer pairs and simple followup questions.
QnA Maker also expose an API which allows me to set synonyms, which really increases the "hit rate". I don't know if Power Virtual Agent makes use of any synonyms when ranking topics as QnA Maker do, but if it does I'm quite sure it doesn't support my language (Swedish) which gives QnA Maker (where I can set synonyms myself) an advantage.