Mike D'Angelo’s review published on Letterboxd :
Always a bit embarrassing when all of my peers are having multiple orgasms and I find myself mumbling, "Yeah, it's, uh, quite good." (Actively disliking an instant classic is easier, somehow—genuinely contrarian rather than just muted.) In terms of writing and acting, this is every bit the equal of Lonergan's previous films; there's a scene between Casey Affleck and Michelle Williams toward the end that's so emotionally harrowing, in such a radically unconventional way, that I'm actually kinda scared by the prospect of enduring it again. And the amount of warmth that Lonergan somehow wrings from crippling, potentially terminal devastation, along with his devotion to quotidian detail in this über-bleak context, is nothing short of miraculous. But Manchester By The Sea* is the first screenplay he's written that demanded a stronger, more visually expressive director than he currently is. There are a number of crucial flashbacks in this film, and they don't appear to have been thought out beyond their narrative function; Lonergan's trying to do something here not unlike what Sean Durkin did with Martha Marcy May Marlene, and he doesn't have the formal chops to make it work. Consequently, large hunks of Manchester, mostly involving the tragic backstory, play like (powerful) detours, which I feel certain is the opposite of Lonergan's intention. It all comes together by the end, so I'm hopeful that a second viewing will mitigate my reservations (and Lonergan may tinker with the film slightly in the meantime, though I don't think he can fix the problem—not that anyone else seems to feel there is a problem—without doing reshoots). For now, though, this is my least favorite Lonergan...and every filmmaker should have such a terrific "nadir."
* Note capitalization—it's the full name of the town (technically Manchester-By-The-Sea), so "By" and "The" should always be uppercase.