Playing down one defender wouldn’t be too bad because the offense is forced to have 5 ineligible receivers by the rules, so rushing only 3 men means you have enough to cover everyone on a pass play. The offense still probably goes for pass plays because of the need of the defense to weaken the rush, giving a longer time for receivers to get open. Playing on the ground wouldn’t really take advantage of the extra person very well, since there’s only so much space on the field where the runner’s going.
Playing down a man on offense probably means you need to run the ball, to take advantage of the fact that you have to field 5 interior lineman and they can theoretically cover all the gaps and just push the defenders up field to make space for the runner.
So I don’t think it’s really quite as bad as others make it out, at least in theory. It definitely though wouldn’t be something that teams would have practiced for, at least not until such a rule was implemented. If it were, I think we might see some interesting strategies that would make a reasonable game out of it, even if clearly the understrength team was at a considerable disadvantage, which is the same as in soccer, really. In soccer when we played a man down we just cut one of the two forwards; this leaves you pretty much as you are defensively while still leaving you with one attacker to try to make something happen.