Don't trust my 85mm 1.8 and 50mm 1.4 primes...

While one can never rule out quality issues with a sample, here is my experience with my copy of the 50mm 1.4

I am only using the center AF pint on my MK II with some micro adjust I did. On the MK II focus is quite fast very accurate and consistent even at 1.4. The lens is overall better than my 50mm 1.8 II in this setup.

Using the very same lens on the 600D of a relative, the lens had significant issues with focus accuracy. Funny enough, my "old" 50mm 1.8 II did very well on this camera then.

Don't know if this answer helps, but at least for me I can deny that there would be a general problem with the 1.4 on all 5D MK II... On a crop body the situation may be different ...