Mickey,
Although I've never hunted NA or S. Pacific game I have hunted (as a PH and for myself) a lot of animals in a lot of African locations. So I'll add my "two pennorth" where I can. If there's one truism about Africa, it's never say never and never say always. I think you're over-generalising.

Although most African animals are pretty resiliant, mostly due to the fact that everything wants to eat everything else. Some animals die easier than others. Nyala for example (generally) die easily.....Assuming correct bullet placement.

There's also large parts of Africa where nearly ALL the shooting is in excess of 200 yards.............. having said that, I personally don't like that kind of shooting. I prefer to get my clients real close. Especially to dangerous game. Also I don't let them shoot from (or near to) the hunting vehicle. It's actually illegal in most provinces of SA, and also most other African countries. And unsporting in ALL countries!!!!(but not always, just generally)

As to harder to hunt? I can't answer that as I've never hunted in the US. But I will say that on average there are probably a lot more animals per square mile in the African bush than in most places in the States or elsewhere. This must mean that the average hunter sees a lot more chances to get a shot than most other continents.