There's a difference between honest and blunt. Very few people react well to direct criticism and go on the defensive.

There are a few tricks you can employ to soften the blow:

Framing a problem in a way that allows the other person to feel like the can step up and do something positive gets better reception than framing it like they should step away from doing something negative. People want to see themsevles as the heroes not the villains.

On a macro level balancing your criticism with positive observations (and leading with them) can help put people at ease and make you appear less of a pessimist/complainer/picky and more thorough.

For specific personal criticism the [sandwich](https://hbr.org/2013/04/the-sandwich-approach-undermin) strategy is also helpful. It's very similar to the one before but aims to improve who your opposite feels.

I agree with others that it's difficult to judge the situation, whether your boss will be open to what you have to say. But I'd say it's probably better to assume good intentions and do what you've been asked to do (give honest outsider feedback). If that isn't actually wanted you're in trouble anyway so might as well find out early...