Borg failed to win the USO or AO in any year between 76 and 81. There's no excuse. End of the day we draw the line and find Nadal stands above Borg.
Borg didn't play the AO from 76 to 81, so "failed" is a pretty strong and almost hostile word to apply in that context, but perfectly reasonable for the USO.
As has been pointed out already numerous times, the AO didn't mean much back in Borg's day, while the YEC's meant a lot more than they do today.
The way I see it, Borg's 3 YEC's are "almost" equivalent to winning 3 AO's in today's era. At the very least, his 3 YEC's offset Rafa's single AO title. But his failure to win the USO is a definite mark against him.
All things considered, I still pick Borg over Rafa, but it's pretty close and I'm pretty sure Rafa will surpass him, in my books, before all is said and done. If it weren't for Rafa's knees, I'd put money on Rafa challenging Laver/Fed for GOAThood. But now it's impossible to say. From my viewpoint, he'll surpass Borg either way, but whether he can topple Samrpas remains to be seen...