You know that this is not true.
Borg didn't think it was the most difficult slam. He thought the USO was the most difficult slam.
Sampras didn't think Wimbledon was the most difficult slam. He thought RG was the most difficult.
Also although you don't count him as a "great", Nadal thought USO was the most difficult for him to win.
for sampras was the most difficul rg like fed or borg the USO because he never could win it.
but in general , wimbledon it`s the most harder , all the players wants to win wimbledon because it`s legendary and very dificcult.
on clay you have time to recover , but in grass if your enemy serve like a god , you are lose.
the other problem in wimbledon it`s the preparation , on clay you have 3 masters1000 , the same in hardcourts , but in grass???? only halle or queens.