I used to offer feedback to rejected candidates... until a person felt that was unwelcome and left us an extremely angered review on a trusted website. I have been careful with words and have tried to avoid confrontational or subjective topics. That didn't seem to matter much to them.

All in all be thoughtful of the person in front of you and try not to be a jerk. Be prepared despite that some people will not want to hear what you have to say.