Nadal's ability to take over matches with his fh has been one of his traits. Haven't you heard commentators, often say that once Nadal starts moving an opponent around, he doesn't let him off the hook with his fh? Look at Nadal vs Murray in his both Wimbledon matches, Nadal completely took over those matches with his fh. At FO, he has consistently taken over matches wit his fh. On hard courts, it's being mixed. I have seen Nadal completely dominate with his fh on hard courts in some matches but at times, it hasn't been he case. Nadal's forehand is monstrous but it can be somewhat neutralized by an attacking player who takes time away from him. What you fail to understand is not that his fh lacks firepower, it's that some opponents rush him...when they rush him, they don't allow him to completely unleash his full fh swing. Nadal, unlike Murray and Hewitt, is not a counterpuncher...he needs a bit more time to fully unleash on a fh. Nalbandian, Davydenko, Del Potro have been able to take ball early and take time away from Nadal. Its not that they are allowing him to hit forehands but he can't do anything, it's that they re not allowing him to position himself to hit many good forehands. There is a difference. For example, eoy masters match where fed bageled nadal had nothing to do with Nadal not being able to do damage with his fh, Fed just didn't allow nadal to hit comfortable fh's by serving well, attacking. Davydenko didn't let Nadal hit many comfortable forehands either, he was always rushing Nadal by taking ball early; Nadal was hitting a lot of fh's rushed and this is where he is vulnerable, he is not a counterpuncher. He reminds me of Wawrinka in some ways; Stan also needs time to unleash his strokes. See how Stan at times looks so bad on faster courts? but so ruthless on slow hard and clay? when Stan is given time to unleash his strokes, he is deadly..when rushed, he looks pedestrian. Have you ever seen stan do well on indoors? on Cincinatti? fast surfaces? why aren't you creating threads making fun of Stan's low winner counts on these surfaces?
in summary, some players don't allow Nadal to set up properly to hit many good forehands, this is a tactic which only a few can employ successfully, on some surfaces. When given time, it's one of the greatest, possibly greatest, shot in history.