Take the best LBs, Free and strong safeties, RBs, Corners, Receivers and TEs and have them line up against and hit the best rugby players without pads, and most of those rugby players will get put on their asses.
If the best rugby players were better athletes, they'd be over here playing in the NFL and making millions. Don't say it's not possible either because it can be done. A superior athlete like Antonio Gates picked up football after playing college bball and is now a pro bowler.