Just yesterday I had to export some old premiere project again that used the ultra keyer.
Funny thing, when I rendered it with using OPEN CL, only halve the frame of the keyed layer got rendered, the other halve stayed blank and showed just the background.
I had to re-render everything with software only.
Plus I can't really tell any speed difference between the two modes. Only difference I see is that premiere barely uses the cpu horsepower during export with gpus enabled. Frustrating.