Hmm not exactly, the southamericans are usually treated with Butantan Soro Anticrotalico and theres also a mexican serum, Antivipmyn, that suppose to be good for mostly NA species.As for them not being more or less dangerous, you need to look at each individual species and it's venom composition - there are so many species of Crotalus and the anti-venom CroFab is used in all envenomations - designed to work for all in the genus, so the effect is different for different species. And venom yield as well.
For northamerican species, most are still treated with Crofab.