I played suburban-level rugby for a few years and suburban gridiron for one. I’ve never played competitive rugby league, because I wasn’t anywhere near fit enough. I played loose-head prop (I was big/slow enough for tight head, but my shoulder would dislocate in scrums so I had to move to the other side) and offensive/defensive tackle in gridiron, although I was a more natural fullback - but our fullback was good at his position. We very rarely had enough players to be able to run completely separate teams on each side of the ball. I’ll also fully admit that I was hopeless at gridiron and still don’t know a lot of the rules.
Let me challenge the idea that in union every player’s putting in for 40 minutes each way. In rugby (union), the forwards are in every play - particularly the props (the biggest, fattest guys on the team). You had to be at every breakdown, and if you couldn’t quite get there you had to be one away from the ruck and moving fastest the instant that the ball came clear.
Wingers, on the other hand, as the lightest and quickest players on the field, had to be careful not to be caught combing their hair or staring at women in the crowd on the four or five occasions in a game when the ball came in their direction.
The equivalent in gridiron is if the two teams’ lines had to play both offense and defense - and three other games, besides - while the rest of the teams got to go about business as usual.
Compared between the two, gridiron was by far the less physical game. 11 minutes of activity feels about right. The lines would tear into each other more fiercely, but I wouldn’t feel it the next day to anywhere near the extent that I’d feel after a game of football - particularly if we were playing a team like South Sydney New Zealand.
Technically, in those positions? Rugby was much more the technical game. Gridiron was about hand and some gross body positioning, but if you stuffed up all that happened was that a defender got past you (or you didn’t get past your guy). In rugby, if you didn’t have your weight and flex just right in a scrum, or if you weren’t co-ordinating right on the lift in a lineout, someone might be in a wheelchair because of you - and that someone might be you. Gridiron might have had more plays to memorise - O.K., any plays to memorise - but the skill level to make sure that you’d be walking off the field at the end of the game was much lower.
Playing where I was, and at the level I was, the hits were harder in gridiron, and almost all on the front of the upper body/head - the fact that you were naturally squared up meant that the contact was more concussive. When I was tackling playing rugby, it was a lot more targeted to the lower abdomen and down, and there were many more hits from the side and glancing shots.
There was also a much greater culture of looking after your opponent in rugby at the level I played. If you hurt someone in a tackle, you’d guard them in the ruck. But if you were a cheap shot merchant or started playing dirty, all bets were off. There was one Christian Brothers’ breakaway I had to stomp the head of after the second squirrel grip he went in one match. I got sent off, too, but the ref cited the wrong player in the match report.
I’ll also challenge that you couldn’t go all-out in rugby. That was a rookie assumption that got a lot of people hurt. You had to go all-out 100% of the time, particularly in anything employing strength, because everyone assumed you would be and if you buckled or collapsed because you were only going in half-arsed, someone would get hurt.
Of course, all of this was at the levels at which I played - amateur adult, and not a high level of amateur. As things get to higher grades, things may change.