You can't compare Rugby to Football. Both are different types of games played at different levels of exertion. Rugby is closer to soccer in that the plays develop organically and the action is close to constant. There are also very few open field runs as most of the time the action is rather centered around the ball, when the ball carrier is tackled it is usually in a closed space. Football is all set plays and moving players through open spaces, esp on offense. The tackles can occur in a closed space or out in the open. Therefore the hits are more violent. You can tackle a rugby player using your arms as it is ssually a tackle that occurs in a swarm of player and the runner is generally more upright when running. Football runningbacks/receivers run in a more crouched down position and usually have more space to maneuver. If you try to arm tackle a running back he will either bounce of or throw a quick juke so that you end up tackling air.
There is no known way to form tackle a person where your head isn't in the lead. It sits on the top of your body, therefore it naturally is the first thing to make contact on most tackles. This is why the most important thing is to keep you neck in a bulled position, this compresses the vertebrae and keeps you from getting injured. Players get paralyzed when there neck is facing down on a hit because there vertebrae are not compressed. Harrison was not atempting to spear Cribbs or Massaquai, he did not throw his head at either player. His force carried his head through either player, he did not (like Merriweather) cock his head back and throw it into the offensive player. If Harrison would have attempted to slow himself up on the Massaquai hit chances are he would have ended up just as hurt or or more hurt than Massaquai. Has Harrison done some questionable things on the field, of course he has (the Todd Heap check on a spiked ball), but this weekend he did not.
Defensive players are already at a distinct dissadvantage. The offense knows where they are going and when they intend to go there. To ask a defensive player to try to judge just how hard he needs to hit is ludicrous.