Not sure about others by my exit interviews have typically been towards the end of my notice period. I've long since communicated (or at least vented) to my manager what I've been unhappy with as well as having a detailed talk upon giving my notice. And likely as a result, my experiences have been HR just nodding along. I certainly can't imagine treasuring up all the negatives and blurting them all out to HR one one meeting as this post games out.