NFL players hit at about 3-4 times the force then Rugby player
This is just absolute garbage. You can't seriously believe this is true. You could argue slight harder overall, but that's totally countered by the fact that apart from a running back, they're probably only involved in 5 or 6 tackles per game
If you get absolutely lined up on toast like with the Clowney hit, there is a shitload of force. That kind of hit isn't particularly common at all, and most tackles would be roughly equivalent to an NRL tackle.
6'6, 120kg guys who can sprint and hit like a freight train exist in the NRL, and every now and then one of those monsters is able to line someone up just like Clowney did.
As you say, players hit with their pads and helmets, but they're also protected by their pads and helmets.