Retrofocus = inverted tele not really wide angle lens designs in the purest sense of the word. A good retrofocus design is not much worse than it's non retrofocus sibling but it's a much more complicated design and more things can go wrong at the assembly or later stages. The Mamiya 6 is a superb camera its 6x7 sister is supposed to have the sharpest MF lenses in existance, since it's mostly used handheld this advantage is often lost. The advantage of the RF ends at the Normal focal length, some Normal lenses for SLRs are way sharper than their RF cousins. Another often stated advantage of the Mamiya RFs is the lack of Mirrorslap which can cause vibrations = less sharp results.
Compared to 35mm SLR lenses the Mamiya is sometimes better and some 35mm Slr are better than the Mamiya. MF lenses usually have less resolution than their 35mm counterparts since the neg needs less enlargement.
A Zeiss, Leica, or some Canon and Nikon lenses certainly have more sharpness and resolution than the Mamiya lenses.
The bigger negative makes up for these shortcomings though and overall the Mamiya will create sharper images with better tonality.