I played MLB in Highschool and transitioned to flanker in college. The demands of rugby overall when you consider both conditioning and strength are far greater than in football. At the professional level, NFL players typically need to be active for about 12 minutes per game. With Rugby, that number is closer to 60 minutes.
Additionally, both of the positions i played for football and the one I currently play for rugby typically rack up the most tackles per game. I was a very good MLB and averaged about 8 tackles per game. In rugby I was averaging 15 or so tackles per game. The top recruited highschool linebackers may have around 15 tackles per game... if they're lucky.
In hs football i suffered no major injuries and only the occasional sprain, strain, or pulled muscle.
In Rugby I come home and sit in an ice bath every day. I have broken my nose several times, been in 10+ fights/brawls on the field, broken arm, ribs, had 15 stitches down my arm, been cleated in the face, and kneed in rucks more times than I can count.
The best way to describe rugby to a non-player would be to imagine placing 30 men on a field and telling them to beat the shit out of eachother.