Lendl won 3 FO in 5 finals. It was a transitional era when racquet technology and game play were being revolutionized and there were no dominant clay court players. McEnroe's game should not have been good enough to get to the FO final, as great a player as he was.
Vilas won one FO in 4 finals over Brian Gottfried. A very good but not great player.
Wilander won 3 FO in 5 finals, including a loss to Yanick Noah. Same story as Lendl.
Bruguera won 2 FO in 3 finals and won 4 major clay titles total in his whole career. Dude, WTF is wrong with you?
Courier also won 2 FO in 3 finals and 4 major clay titles total in his whole career.
Federer has won 1 FO in 5 finals, losing to the best clay courter of all time 5 times! Since before he won his first GS, Federer has only lost to 3 times FO champion Kuerten, 6 times FO champion Nadal, and the only player to beat Nadal at RG, Soderling. If it weren't for Nadal, Federer could have been tied with Borg for most FO titles with 6. Federer is clearly a superior tennis player to all of the champions mentioned previously. His career record and all-court prowess is definitive proof. That he does not have more triumphs at Roland Garros does not diminish his status as a legendary clay court player, rather it elevates Nadal as an even greater champion on the surface.