It's a fine line.
Feedback is welcomed by the recipient and may help to strengthen a relationship.
Criticism is a step on the path of destruction (for a relationship).
There is only one tiny factor that separates the two:
Did the recipient of your opinion ask for it?
Yes = you are giving feedback
No = 99% of the time you will be perceived as giving criticism ... whether the recipient says it to your face, or not!
When you feel the overwhelming urge to 'give feedback' and offer your opinion, ask yourself:
Did this person ask for my opinion or honest feedback?
So, ask them:
“Would you like my honest opinion?”
When they say ‘No’ – if you want to keep your relationship - respect their decision making maturity and keep your opinion to yourself.