Literature on the learning benefits and interpersonal mechanisms of peer assessment (PA) and teacher assessment (TA) has been inconsistent. As part of a large-scale study, the research reported here has addressed the effect of formative PA on language grammar uptake and complexity, accuracy, and fluency triad scale levels, in comparison both to TA and to having no formative writing assessment. Thirty-nine proficiency-matched adult language learners in a nonprofit language institute attended eight writing treatment sessions and completed two versions of a grammaticality judgment task and writing task at pretest and posttest conditions. Following the experimentation, a semistructured focused group interview was conducted to elicit their perceptions toward feedback experiences. Within-group results revealed that accuracy and fluency levels of PA and TA groups significantly improved, but other target skills did not. Focused group results indicated that participants were in favor of teacher feedback rather than PA due to trust and psychological safety issues. Results support the importance of considering the match between learners' social, cultural, and religious background and assessment approaches in order to maximize acceptability and utility. Moreover, several recommendations for improving the quality of PA in language classrooms are supplied.