Being too blunt raises defenses and completely wipes out the effectiveness of your feedback. Folks that are invested in outcomes make choices for good reason, and they've probably got a track record to back it up. You have to meet them where they are, and considering their communication styles and how they make decisions will improve the chances you're actually heard.

It's a fact of life that people shut down when approached with evidence that refutes their world view or choices. It doesn't matter if it's your boss or grandparents.