McEnroe's groundstrokes are very compact and simple. Because of that, they probably demand less innate ability than modern strokes.
I'm more impressed by Federer being able to hit on-the-rise than McEnroe, especially on the forehand side.
McEnroe, Seles, and Agassi could do it easily, mostly because of their stroke mechanics. I should also mention Date-Krumm, because her strokes were the opposite of "natural", yet they are highly effective.