I only had experience with the EF 85/1.2L v1 and the 135/1.8 is much better than that one in pretty much every parameter. Not only all AF parameters, but the EF 85/1.2 had horrible wide open LoCA/color fringing, which I have not seen even a trace of from the 135/1.8. Its AF performance is more similar to the EF 85/1.4L IS but with maybe slightly worse focus consistency. It could also be a focal length/depth of field equivalency situation, where 135/1.8 might be more challenging for AF due to shallower depth of field and the relatively greater challenge of non-linear subject movement. (85/1.4 has an entrance pupil of 61mm vs. 75mm of the 135/1.8) It could be more difficult for AF to keep on top of with a single nano-USM, vs. dual nano USM in the 70-200 Z, or perhaps the new for Canon VCM (voice coil motor) drive now appearing in some RF lenses.
By no means is it terrible or unusable for AF. It's just not as 'sure thing' as most other RF lenses. It's maybe like 85% consistency vs. 98%. I still always get something usable.
Another weakness of the 135/1.8 is stronger tendency for veiling flare in backlit situations than I would have expected for a lens of this class. It actually can be kind of annoying because I like to shoot backlit a lot and sometimes a bit of flare can be appealing. But sometimes the veiling flare is stronger in the image periphery than in the center, like a reverse vignette effect, which I absolutely do not like.
