He had a few games where his defense was solid, but for the vast majority of the season he really struggled to guard offball and had a very slow first step losing quicker players regularly. Justin's metrics have him grade out as the worst defender who received regular minutes. And eye test at least for me agrees. He was a subpar defender in my opinion and while adjusted defensive plus minus efficiency stats certainly aren't a be all end all, we were giving up 1.084ppp whenever he was on the floor, meaning we were around the 220th best defense efficiencywise when he was playing.
By comparison, while the teams were much much different, the team gave up 0.875ppp while Sencire was on the court last year. Adjusting for individual metrics, Sencire had one of the Top 10 largest defensive impacts on a team in the nation were it to translate to a per 40mpg basis.
Similarly on a per 40 basis, Sencire vs. Harmon translates to Illinois giving up around 14 less pts per game, which while very imperfect from a statistical perspective is still an incredible number. Basically, comparing Harmon to Harris defensively is just not remotely close.