That’s not a real request. People rarely want the truth when they want your opinion.
When people make that request, they really want your support and confirmation of their effort in a realistic way. Give that to them.
The most powerful way to influence people is to influence the story they tell themselves about themselves. If you think someone’s work sucked and told them that, you crush that story.
Instead, try to inhabit the person’s worldview. What might have been important to them? What details might they have focused on? Comment on that. Say what you noticed.
Being indirect and not saying what we truly mean is an essential — albeit frustrating — part of what makes us human. You do it. I do it.