Some people offer certainty because they have not stayed for the consequences.

Certainty is cheapest for the person who will not be present for the outcome. The consultant leaves, the visitor flies home, the commenter closes the tab, and none of them will be there when the plan meets its first hard week. People who stay with the consequences tend to hedge, qualify and mention what could go wrong. That sounds weaker and is usually worth more.

If advice arrives with unusual certainty, ask who carries the result if it is wrong. Weight opinions by that answer rather than by how sure they sound. Give the same courtesy to your own strong views about situations you will walk away from.