I have a tendency to stumble over my words or forget what I was saying mid-sentence (yay ADHD). Once you have to move the cursor backwards, typing will always be faster than speaking.
The biggest issue I have with dictation currently is that as I'm speaking it'll type out what it thinks I'm saying and then revise that what I actually said as it goes. This results in me getting distracted reading the words appearing on screen and losing my train of thought.