Despite the wide use of peer assessment, questions about the helpfulness of peer feedback are frequently raised. In particular, it is unknown whether, how and to what extent peer feedback can help solve problems in initial texts in complex writing tasks. We investigated this research gap by focusing on the case of writing literature reviews in an academic writing course. The dataset includes two drafts from 21 students, sampled to represent a wide range of document qualities, and 84 anonymous peer reviews, involving 1,289 idea units. Our study revealed that: (1) at both substance and high prose levels, drafts of all quality levels demonstrated more common problems on advanced writing issues (e.g. "counter-argument"); (2) peer feedback was driven by difficulty of the problem rather than overall draft quality, peer comments were not well aligned with the relative frequency of problems, more comments were given to less difficult problems; (3) peer feedback had a moderate impact on revision, and importantly, receiving multiple comments on the same issue led to more repairs and improvement of draft quality, but consistent with the comments received, authors tended to fix basic problems more often. Implications for practice and research are drawn from these findings.