People say that full western grip is weaker against low slice.
Full western grip means you have the topspin to hit low balls as hard as you like, because the spin will lift the ball high over the net and still bring it down. Meaning you can be as aggressive as you like on even the most perfectly hit slice (which is a low percentage shot as a perfect slice must travel very low over the net, so the slicer will make more errors than you anyway)
Additionally full western is easier on high balls.
So Nadal is in his comfort zone hitting extremely high balls and extremely low balls and everything in between.
There is no way you can break down the Nadal forehand.
You can outpower Nadal's forehand, bring him to his knees. It is not like if he can use pace like Federer right back at whoever is bombing his forehand with flat power strokes (like Del Potro). Nadal can only defend with forehands going high up on his opponents backhand when dealing with power strokes, a predictable pattern that can look very weak against anyone in form and who is tall (Del Potro, Soderling) or have a great backhand (Murray, Djokovic, Nalbandian).