I use it all the time... It’s good but not THAT good.
The key for it transcribing a played audio file is the speed of the voice recording the file. The faster the talker, the more verbal stumbles and pauses, lisps or mispronunciations, the more errors.
Try stopping the played audio between pauses. Otherwise it get’s behind, leading to more errors.
Like autocorrect, some of them can be hilarious.
I have Dragon NS 12. Here’s a video where a guy explains how to do audio file transcription in Dragon NS 13:
https://www.youtube.com/watch?v=M9V6jg0JFZI&t=6s