I don't know man. I play lacrosse which sounds loosely similar to rugby in that aspect. Handful of plays and formations, but mostly individual decisions and knowing where to be and when, etc.
Football playbooks are literally hundreds of pages long, a quick google search yielded 600 pages as an answer, with an absurd amount of formations. That, combined with the different variations of each play, and the quarterback's ability to call the plays at the line of scrimmage, along with audibles, makes me think that learning the game of football is much more difficult than you're giving it credit for.
http://www.si.com/extra-mustard/201...it-memorize-nfl-playbook-neurologist-explains
I'm in no way knocking Rugby, I'm just saying that for how similar the games are, they are completely different from a mental standpoint.