Feedback works when it creates personal credibility, when it gets people to realize that we have valid, useful observations about them. Observations are not judgements, assessments of good and bad. We simply describe what we see others do and the consequences of their choices.
It works when we give people feedback they agree to receive in the way and when it is most useful for them.
It works when we exercise the humility to ask for as much feedback as we offer.
It works when it strengthens rather than weakens their confidence in their goodness.