Because it's a thin solid rotor as opposed to the thicker vented rotor the Cobra uses and the 12" Baer vented kit.
The non-vented rotors retain heat more and don't resist fade as well. Also, a lot of guys have had issues with noise in the forms of clicks or scraping sounds from the. This may be from the slots, or it also may be from the fact that you have a bracket, bolted to a bracket that is bolted to another bracket, so lots of places for harmonics to transfer.
Finally the e-brake tends to work poorly because the cable is stretched out pretty far.
If you want bling bling, go for it, but for performance, either go with the factory OEM Cobra 11.65" kit, or the baer 12" rear kit...which is considerably more money.