I think you have to differentiate between professional (or collegiate) level football v. less formal games.
Having grown up playing lots of American Football all through school (for fun, not competitively), I think the rules are a lot more of a spectrum. For someone to play for fun or even watch the pros, most of the rules don't really affect the overall understanding. There will be some plays that get reversed or penalized on some weird technicality, but it's relatively rare. Things like "offsides", "false start", "delay of game", "intentional grounding", and personal fouls seem like the most common infractions, and those aren't really all that complex once you understand the basic mechanics of plays like the system of downs and line of scrimmage. "Illegal Formation" and various others get ridiculously technical and complex, but unless you watch a lot of football (and even then) it's not something that will have much impact and the refs/commentators nearly always explain what the infraction was.
Now that said, I don't mean to undersell the difficulty in learning the simple structure. Trying to teach my kids the rules when they've never played an informal game and were watching NCAA games with me was a helpful exercise at appreciating the weirdness. It's not the most intuitive for sure. If I hadn't grown up with it and had that informal experience as a baseline, I'd also struggle to make sense of the game just watching it without much explanation.
Having grown up playing lots of American Football all through school (for fun, not competitively), I think the rules are a lot more of a spectrum. For someone to play for fun or even watch the pros, most of the rules don't really affect the overall understanding. There will be some plays that get reversed or penalized on some weird technicality, but it's relatively rare. Things like "offsides", "false start", "delay of game", "intentional grounding", and personal fouls seem like the most common infractions, and those aren't really all that complex once you understand the basic mechanics of plays like the system of downs and line of scrimmage. "Illegal Formation" and various others get ridiculously technical and complex, but unless you watch a lot of football (and even then) it's not something that will have much impact and the refs/commentators nearly always explain what the infraction was.
Now that said, I don't mean to undersell the difficulty in learning the simple structure. Trying to teach my kids the rules when they've never played an informal game and were watching NCAA games with me was a helpful exercise at appreciating the weirdness. It's not the most intuitive for sure. If I hadn't grown up with it and had that informal experience as a baseline, I'd also struggle to make sense of the game just watching it without much explanation.