Would an NFL running back dominate in rugby?

[WhoIam]

I'm an American visiting Australia and watched a full rugby game for the first time last weekend. I admittedly know nothing about the sport, but I was struck by how weak the ballrunners were, at least compared to American football. Defenders seemed slow and it seems like a fast outside back would be able to dodge most of these and be able to score a ton of touchdowns (or whatever they're called.) An inside power back also seems like he would be able to get a lot more tough yards than the comparatively weak running I saw. Am I onto something or here or am I missing something big about the mechanics of the sport?