I disagree. Most clay courters arent contenders for big titles on hard. For instance Coria and Gaudio dominated on clay but were never considered threats on faster courts. Very few clay courters out there have any chance for a major title on hard courts.
Gaudio I'll give you. He cant really do much outside clay.
But Coria? Cincy QF in 03, IW QF in 04, Miami FINAL in 04, USO QF in 03, won Basel in 03, beating Ljubo en route, and he beat Moya indoors at the 03 TMC. USO QF in 05, 05 Beijing final, beating Tojo, not a bad hard court resume
and what about Ferrero? or Guga? or Rafa?