Loeb was extremely fast with active diffs. Bosse was extremely fast with active diffs. Just as they are without them.
There is also age, motivation and bad luck. When looking at Mäkinen last year more in detail it actually was not that bad at all, but he was haunted by mech problems throughout the season.
A good driver is a good driver regardless of his machinery. he adopts to different surfaces and cars very fast. That is why I, for example, put Ekström very high on the list of good drivers.