Rugby (union) isn't like anything else, and often comes down to very minor and complex details. The differences with American Football are big, because you are talking about guys playing in constantly rolling phases of play across a field (kicking for territory, passing phases, mauls, rucks, scrums, lineouts) sometimes for minutes at a time. That is going to be different to a sport where plays come down to seconds and everything is about being explosive in short bursts around numerous breaks in play.
In rugby you get hurt and tested through sheer constant and exposed physical contact and aerobic endurance with guys ranging from 14-20 stone, mostly with body fat percentages below 20%, as opposed to getting stop-start monster hits from a 17-25 stone mountain in pads.