What, specifically, is 'weak' about the 50mm f/1.2L?I'm guessing it is the relative lack of sharpness in border regions on full frame, from a quick glance at Photozone results. Seems to fall off significantly there. Not an issue on crop sensors. It doesn't sound like an issue in real world shooting, since if you're getting the f/1.2, you're wanting the current shallowest DoF at that length and unlikely to get everything sharp in frame anyway. There is the even older f/1.0 it replaced that's even softer.
I suspected that might be the perceived issue. Everything in photography involves tradeoffs and compromise. Want a faster shutter speed in low light? Fine, open the aperture and sacrifice DoF, or raise the ISO and add noise. Tradeoffs. In the case of the 50/1.2, Canon's designers chose to produce a lens with superior bokeh. The way to do that is to intentionally undercorrect speherical aberration, meaning sharpness is reduced a bit. But the lens is intended for portrait work ("Taking advantage of shallow depth of field allows the photographer to capture subjects with impact, such as for wedding portraits," from Lens Work III) - as such, especially with such a wide aperture, bokeh is critical - and in most portraits, it's ok for the resulting image to lack razor sharpness (all the portrait picture styles, from Canon and in 3rd party software, have the sharpness set lower than for general shooting for a reason).