In Art school we learned to take and give constructive criticism only when it's ASKED for. I feel like it's a skill that has to be trained and learned and no one really talks about it because since most consider our works as /personal/ pieces, sometimes any form of opinion or criticism is taken as a direct attack or judgement on ourselves.
Going back to the question, again, if one ASKS for it is okay. On the other hand, if the person wants to give it, they have to offer and continue if the author gives them permission to. If not, pls just shut the up.
Unwanted opinions will never be welcome, of course.