You are entitled to give honest answers in your exit interviews, but do so without burning your bridges - relationships with boss and co-workers. For example, in my only exit interview so far, I said I had no growth prospects in my present job, but my boss and co-workers were very good with me.
Even if you are leaving because you got a better job, just be diplomatic. No harm being cool about it and moving on effortlessly. This is life, boss. Something else will work out. This wasn't just meant to be.
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