8 out of 12 slams on clay; 18 out 24 MS on clay; 13-2 H2H against Federer on clay (7-8 on other surfaces); didn't win a single title off clay during a 21 months period; as a matter of fact 13 of his last 14 titles have been all in clay (and this during his so-called peak years); I could go on and on.
A player with such an unbalanced resume could never be considered G.O.A.T. if such thing existed ... so yes nothing to discuss here!
Nothing more that needs to be added really. And Nadal is a great player, one of the greatest. But he hasn't shown that off clay, at least on a consistent basis. 15 titles out of clay just doesn't cut it. Sorry.
The stain on Fed's resume will probably always be that one thing. Nadal's h2h.
Nadal's stain is his inconsistency out of clay. That's all that needs to be said.