I suspect there are few concussions in rugby since you cannot spear, cannot tackle above the shoulders and you have to roll off the ball as soon as you are on the ground and you can only go into the pile from the 180 degrees on your side
Plus in rugby you have little equipment that can be used as a weapon, like the helmet. And rugby seems to be seriously concerned about the concussions that might occur, requiring mandatory checks any time a player might have possibly had “his bell rung”. They also are changing the rules to reduce the chances of concussions.