Prone to bias because of quality of competition. Nadals not racking up more weeks #1 than Fedovic if Fed’s quality of competition during his prime was declining Roddick, Hewitt, head case Nalbandian/Safin and Djokovic’s was post prime Murray, Nishikori, Raonic, geriatric Fed.
Atleast you can dissect slam H2H by surface/year so that if you wanted to you could look at prime v prime on a neutral surface. It’s not a perfect metric but more reliable than weeks #1 in my opinion. I think this is a more important indicator anyways because it demonstrates performance against the best of the best at the pinnacle of tennis.