JJ Watt, Jason Pierre-Paul and Demarcus Ware are Defensive end, they should compare players of his size as:
Jim Hamilton 2.02 m and 124 kg
Eben Etzebeth 2.02 m and 123 kg
Kane Douglas 2.02 m and 123 kg
Brodie Retallick 2.04 m and 121 kg
Ali Williams 2.02 m and 120 kg
I say that players like Julian Savea or Israel Folau could make american football plays simple as: Run Straight and Run Wide. In those plays don't have to be a good receiver because the quarterback puts the ball in the hand of the runner. It's not that hard to do, and the strength and speed they have those players I named, they could do it without problem. Or you think that american footballers are stronger than rugby players?
regards
If I were to compare those players to bigger, less athletic guys you mentioned, then it would be even wrose. I compared SBW to bigger guys, simply to make the comparison easier (for SBW - in terms of speed and athleticism that is).
I think the specific guys you mentioned, Savea, and Folau, would make great NFL players. At the same time though, WR and RB (RB especially so) are amongst the easier positions to translate to. Even then, you have to be aware if your team is running a zone blocking scheme (offensive linemen), know the verbiage that goes with that, know the assignments of your teammates, be able to study the opposing team's defensive front, and play based on what you perceive the defense will be running (based on personnel and tendencies from game tape).
Like any player that transitions to the NFL (whether it's from college or other sports) - no matter how great an athlete you are, it's going to be how well you pick up these schemes. Can Savea/Folau (or anyone in rugby do it? Sure - but you have to have the right mix of athleticism and brains.
I'm sure someone like DC or Conrad can pick any scheme, offensive or defensive (I actually think if Conrad was either faster or bigger - or both) - he'd make an awesome LB or Safety.
To the point - it's not as simple as handing the ball off and running - it can be, and sometimes if you're awesome (like Adrian Peterson or Barry Sanders) you can solely rely on that. But not everyone (or most for that matter) will be able to do so.
Regarding strength, in general yes. But I still think that depends on position. BUt largely, yes. Just look at combine stats of bench numbers. While I think there are a number of rugby players that will match or exceed some of those numbers (namely someone like Spies, or the Franks brothers) - for the most part, statistically speaking - the average will be higher in the NFL.
But there's a reason for that. Which is also why it;s a different sport - different needs and different uses for largely similar athletes - it's just the higher end of those more elite athletes, tend to reside in the NFL. I say it's largely due to rugby's relative infancy in professionalism coupled with MASSIVE discrepancy in pay packets (also, having fragmented club leagues and a reliance on even more fragmented national leagues and competition timeframes)
"the best of the best" aren't able to play each other more frequently.
But that is another thread.