Funny, because isn't that exactly what the 50L is?
I rented the lens twice before buying it, different copies, all worked 100% well on my body. No MFA what so ever. When I bought the lens, same story. I guarantee there are more Sigma bad focus threads than 50Ls. The 50 design was inherently flawed and required understanding of this to achieve correct results, where as Sigma generally is a gamble and many have called it the "Sigma Lottery" for a reason. That said heard plenty of bad stories for every lens, since nothing is perfect. However, Sigma is notorious for focus issues and experience so myself first hand.