Mine is vertical at an easy reach- elbow bent about 90 degrees, so ergonomically best for me. I had to go vertical because I went with an 8 inch pull for more leverage.
I just posted a pic on another post but can try when I am on the computer

On my shaker style cabinets it is horizontal and placed on the center stile. I have no difficulty pulling out the cabinet. It works just like a very tall drawer.

Mine is also on the center stile.

Another one that's horizontal on the center stile. I think with a pull out that tall, it makes more sense to have the pull closer to the center to put less stress on the door and hardware. Plus it's a comfortable height to grab solidly.

These pictures are very helpful. One of my concerns was the center bar seems low on ours - but when I see Williamsem and May Flowers pictures those look at bit lower than counter height also.