tl;dr It is my opinion that only God can and should judge people, and people should not give "unsolicited constructive criticism" to other people (or especially about other people behind their backs) and should only give helpful advice when directly asked or If they are a teacher and have told people that they are going to give a lesson and welcomed people to listen to him or her give that lesson if those people want to listen but still respecting the freewill of those people to choose not to listen
anything more than this is essentially slander (and gossip if it is behind their backs) and engaging in sinful codependency