Supercritical sections, like everything in engineering, are a compromise. As I have stated, their purpose is to allow lower sweep angles on aircraft which fly in the high subsonic range, which is a very worthwhile purpose in itself. This is what they are optimised for, and as such, they deviate from what would be the optimised design for a lower speed aerofoil. If they worked better than a low speed aerofoil, even at low speed, then everybody would be using them, from gliders upwards.
I'm not an expert in aerodynamics, and I know very little about the aerofoils themselves, but I do know that when selecting an aerofoil, like anything else, you can't have everything.