Actually, I have owned two specimens, and tested another 5 (actually a few more by now, but lost count a bit ), plus a 50 F/1.8 Mk I, the one with the metal mount and angry wasp AF engine. I have never found any focusing problems with either of these.
However, make sure you focus on a contrast transition, and they will focus just fine, plus give them a little more time than, say, any fast USM L lens. Note that with MF in the good old days we used a centre point wedge, but always on a contrast transition so one could see where one was focusing. That is the concept this lens has really built upon, so it needs to be treated in a similar way, just that MF is replaced with automated movement of the lens for focusing. And focusing on something with a single colour, f.e., and no large enough (size wise) contrast transitions at all, will make it hard on the camara AF system, and because the focusing engine has bigger steps than any USM engine, will make it focus on something that is only close, but will not be focused perfectly.
What will help as well is using the *-button for focusing, rather than using the shutter button. That way it will not refocus when you try to take a photograph, and "get confused" because it is relatively slow and will try to refocus.
A friend of mine, who took over my last specimen, uses this lens for concerts and the like amongst others, in rather dimly lit establishments, on an older camera body, and he gets perfect results each time. I did show him how to use it, according to what I described above, however.
Based on my tests, I rate the 50 F/1.8 Mk II above the 50 F/1.4, because it is usable at F/1.8, and becomes excellent by F/2 or F/2.2, from an optical POV, while the F/1.4 only becomes so at F/2.8. That's my experience, BTW (and I tested 5 specimens of the F/1.4).
HTH, kind regards, Wim