On grass beeing can be neutralized by your opponent just by one shot: the serve.
On clay this is not impossible, so the better player is much more likely to win, much less room for an upset.
On hard the competition is the hardest bacausr everybody is a hard specialist today.
So if anything french open it is the easiest slam to win if you are the best player
Take Nadal out of the equation on French Open winners. You'd have Borg as the one who has win the most tittles, with 6 of them. Now, take Federer out of Wimbledon winners. Well, you'd still get Sampras with 7 and Borg with 5 tittles over there. So, why not as many people have won RG before Nadal?