I wouldn't say Isagi is nerfed like people are saying, more like he's at the top of his abilities and it's even playing with the other two while it's just short of Rin and Shidou.
After all if we remember the stats, Rin is basically a better Isagi cause he has the better phsyical capabilities and way more experience. Isagi COULD be as good as Rin, but he needs way more physical training since he's barely been scraping (as is the way with shounen manga XD).
