I think with Borg's defeats to Connors on har-tru at the US Open in 75 and 76, the fact that they were playing the US was probably more important than the surface. As we all know Connors was considerably more formidable in the US, regardless of the surface, than he was in the Europe (he still had an excellent record outside the US of course). And Connors only beat Borg once in Europe.
Har-tru courts are pretty much exclusively used it North America (though you can find a few in London). These are hypothetical scenarios, but put the pre-prime Borg of 75-76 against Connors on har-tru on Europe, and he would stand a much better chance of winning than he did in the US. Likewise even if the US Open had used European style red clay for a bit, I think Connors still would have found a way to win the title on it. Stich is another player like Connors who was outstanding in his home country regardless of the surface, as highlighted by his excellent record in German tournaments on clay, carpet and grass.