I use one...it works great, and I find it very efficient. Depending on the ram style of choice, it can be faster, too. Never had a problem with knotty wood, and they are typically setup with a bar to prevent wood from spinning. Most of the vid clips I've seen scare me because you see folks using them poorly...wood should be put to the cone on it's side, and depending on thickness, 1/4 or 1/3 of the way up the side of the log. Worst thing to do is stand it on end and hit a big log square in the center. They are nice because they use the tractor for power and transport, have adjustable height from the ground in case you do not want to lift large logs, and have no additional engine maintenance and take little space for storage.
The gap in safety is the lack of a dead man's switch. Rig up a kill or PTO interlock switch to a lanyard or a pressure switch beneath a board to stand on, and you have that covered.
The danger I see with these, vs other types, is that it is a heavy, sharp object when not in use. I screw on a small piece of wood when I'm done (by hand...letting the PTO pull one on and killing power can make for an awful tough startup), so noone can run into it, and so if it happened to fall it cannot pierce.