I would take Ken Rockwells opinions with a grain of salt. However, the good people over at Photozone
tested it on a full-frame Canon camera and it fares quite well. Slightly more distortion and vignetting then Canon's own 40/2.8 STM
and also less resolution, but it has more or less the same distortion and vignetting as the Zeiss ZE/ZF 50/1.4
with better resolution. So, a decent performer.
There is of course more to a lens then these numbers, but they at least show that the basics are good but not spectacular. Now, maybe somebody who actually used it could comment on handling and rendering