I don't know that anyone is saying the IQ of the 24-105mm on APS-C is not good, it's just that there are other lenses that are slightly better. I've tested both the 24-105mm f/4L IS and the 16-35mm f/2.8L II on my 7D, against the EF-S 17-55mm f/2.8 IS, and while the first two are very good, the 17-55mm is slightly better (in addition to being a more generally useful focal length than either of the L lenses since one lacks a wide AoV and the other lacks a telephoto AoV).