You misunderstand. I am not saying all things were equal between when Borg and Nadal played, but that all things were equal for Borg and the players he faced, just as all things were equal for Rafa and the players he faced.
In other words, Borg's opponents played with the same equipment that he did, so any deficit he had, so too did his opponents. The same with Rafa.
You are talking like Borg played with wood vs. guys playing with graphite. Borg had no deficit, no penalty that his peers didn't also have. It was the context of the game at the time.
Now what you're saying about the difference in equipment (not to mention other factors of the tour differences between the 70s and 2000s) has some bearing on comparing stats across eras. That's why we can't say that Borg "only" ever winning two Slams in a year is automatically lesser than the Big Three's three Slam years...Borg only played the AO once, and before (1974) he reached his very best level (1978-80). This is one of the many reasons why it is difficult to compare players and seasons across wide eras.
But we can say, "How dominant was Borg relative to his peers, and how does that compare to Rafa relative to his peers?" Not an easy question to answer, but the key is comparing them to their peers, not to each other.
