When I cite a paper, I don't include a sentence saying whether the paper is good; I say why it is good, because the author of that paper is likely to be reviewing my paper, and I need to acknowledge his strengths before differentiating my work, or he'll kill my paper in the review process.
My advisor taught me that the point of a citation is not to give your opinion on which other papers are good or bad; it's to tell the editors which other scientists should be on your paper's review committee. The way editors choose reviewers is to look through the citations, find people cited who they know, and then ask those people to review the paper.
Thus, the author's goal when citing other work is to (1) highlight potential reviewers who would see your work favorably, and (2) say something positive about them to put them into a mood to see your work favorably.
There is nary an incentive to express negative sentiment in citations.