Flag football is... a really weird sport. It's a super stripped down version of American Football where only four positions from "regular" American Football are actually represented - quarterback, wide receiver, defensive back and safety. There is a center and a rusher in flag football too, but those positions bear very little resemblance to their American Football counterparts. So if you're an end, a tackle, a linebacker, a guard, a center, a tight end, a half back, a full back, a kicker or a punter there is simply no place for you in Flag Football.
The best analogy would be Rugby 7s, except Rugby 7s is more similar to Rugby Union than Flag Football is to American Football. I think most NFL players would struggle here, outside of the four positions I mentioned above (and even then, there would be a big adjustment).
If Rugby 7s had a smaller field, no lineouts, no scrum and no kicking game then what Rugby 7s is to Rugby Union would be a good analogy for what Flag Football is to American Football IMO.