Retrofocus wide angles can be made just as sharp and well corrected for certain aberrations as RF lenses, but they require significantly more complex designs and usually many more elements (which in the past could have led to reduced contrast but is not currently much of an issue. In theory aspheric elements take the place of several spherical elements in the correction of things like spherical aberration.
Originally Posted by David Lyga
For me, the Achilles' heel of retrofocus wides is geometric distortion. By definition a retrofocus (reverse telephoto) lens is not symmetrical, which introduces geometric distortion, which is complicated and expensive to correct. As a result while many RF wides exhibit minimal and/or virtually invisible amounts of barrel distortion, a partially corrected but still very visible amount of simple barrel or complex (mustache) distortion is usually considered a good performance even in a $2,000+ SLR wide. There are a few exceptions around, but generally it seems to me most new SLR prime wide designs are pretty lackluster when it comes to geometric distortion correction. My guess is that is because most people are shooting digital and simple geometric distortion can be largely corrected post-capture. Unfortunately while there are some very sharp SLR wides, in my experience the correlation between price and distortion correction is relatively weak.