Regardless if Nadal beats Federer's amount of grand slams, he will always be regarded as a clay court specialist as he has only won 4 grand slams outside Roland Garos.
And I strongly agree with Roger's quote there. He has the best game play in the history of tennis, his ability to work his opponent in the rallies, draw him in, hit the correct right flatter winner at the right time is why he is what he is.
If you look at murray, nadal and djokovic's game, it is just intense base line rallies with a lot of top spin, it looks impressive to the amateur but one must consider their games are high margin so you are going to get longer rallies.
This is further evident by examining performances indoors like in the world tour finals in east london. Indoors there exists no extra factors like wind, rain so it is only pure tennis. The high topspin game has been showed to be ineffective as Federer has dominated that in the past few years, shows his game under the right conditions is absolutely unsurpassable.