I own both the 50L and 24L lenses and use them handheld 99% of the time.
I will be the first to state any OOF photos are my own user error.
It is all important to remember that you are often smarter than the camera, the AF is based on contract, even with center-only.
If your 'subject' doesn't have enough, it would focus properly either.
Here are cropped test shots for MFA I did for my 50L all on a tripod outdoors.
While a bit off topic regarding MFA, these show how even on a tripod, a little change in focus wide open can blur the while subject.
These are shot at about 4-5 feet away. The center point is just BELOW the red nose. A real-world torture test for the AF system.
Here is the original middle shot uncropped.IMAGE LINK: https://flic.kr/p/Dh5z1o 50L f1.2 best
by James Patrus
, on Flickr