The phrase "damning assessment" means a critical evaluation or judgment that strongly portrays something or someone in a negative or unfavorable light. Full definition
It is a seminal work on the occupation and one of the most damning assessments of what being an occupying force does to a country. (politics.co.uk)
The Chilcot report is one of the most damning assessments ever made of a British prime minister. (politics.co.uk)
The fact he is not is probably testament to the stream of damning assessments coming in on a daily basis from eminently reputable sources. (politics.co.uk)