I think it's more than just all of the Canon 50mm's that suck. I would go as far as to say that virtually all the 50mm lenses available for Canon or Nikon aren't very good, at least above f/2.8. The performance of the Canon 50mm: f/1.2, 1.4, & 1.8, Sigma 50mm 1.4, Nikon 50mm, 1.4D, 1.8G, & 1.8D and Zeiss 50mm 1.4 are all fairly bad above f/2.8.
The Voigtlander Nokton 50mm 1.4, Zeiss 50mm f/2.0 Makro, & Nikon 50mm 1.4G are decent at f/2.0 though, but leave a lot to be desired compared to what we're used to with top lenses at f/2.0.
The whole 50mm fast lens range seems to just be bad.