I think the biggest criticisms are his ability to evaluate and develop QB's. In his 3 seasons as OC, his QB's were Ian Book, Jack Coan, and Tyler Buchner/Drew Pyne. Book was entering his 3rd season as a starter, Coan was a grad transfer who was already developed at Wisconsin, and Buchner/Pyne were players he recruited. Heck, it took him 6 years as a ND coach to have a QB he was the primary recruiter for win a game as a starter at ND. That's not a particularly great look.
ND Offensive rankings with Tommy Rees as OC
2020: 33.4 PPG (30th), 448.5 YPG (26th)
2021: 35.2 PPG (19th), 426.2 YPG (45th)
2022: 30.7 PPG (52nd), 382.7 YPG (73rd)
If they bring in a quality grad transfer QB that has already been developed, which I suspect they will, I think he'll do a quality job. If they somehow strike out and are left with Buchner/Angeli then I don't see how people can be all that optimistic.