The level of NFL specialization will be a challenge. Each side has unlimited substitutions, and has a complete offensive and defensive team as well as a special team to play. For many NFL players, there is no need to be able to pass the ball, only to receive the ball-and the shape of the ball is completely different. For others, there is almost no need to touch the ball. All players on the championship team need to be able to pass and receive the ball. The kicker is a professional role in the NFL, and in rugby games, three of the positions require excellent kicks (scrum in an open game, at least another 2 hand kicks are completed.
The Rugby League may be an easier game to play in terms of learning methods, but compared to the NFL, it has a championship cardiovascular fitness requirement—and higher than the Rugby League. A 60-minute NFL game takes about 3 hours to finish, and there are many personnel changes. Many NFL players are not suitable for playing rugby rules at all. The minimum health requirement is to play 40 minutes in a row and then play 20 minutes after a 15-minute intermission. The league’s requirements for physical fitness are particularly high, because the ball spends a higher proportion of time in the game. As far as I know, many NFL players need a year of physical training to play football to any decent level.