ISIS is at least the equal of Nazism when it comes to inhuman viciousness. But they aren’t remotely equal to the Nazis when it comes to organizing a government or a society. ISIS is a human ebola virus that will quickly kill its host then die itself.
What made Nazism so dangerous is that it was grafted onto a large, modern, successful nation with a history of fielding powerful armies. Germany had a first world economy that could crank out world class weapons and invent new ones. The German general staff was the equal of any of their rivals.
Even if ISIS conquered the whole arab world, which they won’t, they would be trying to hold together dysfunctional and backward societies that have no history of cooperating with each other.
But you see, the islamic world came about precisely in that manner. A small group of depraved criminals following mohammed managed to conquer, bleed dry, and ruin the ages-old advanced civilizations around them. They were hated by everyone too. And after 1400 years the muslim world is still around, despite the dysfunctionality. Perhaps some demonic force holds it together. Along with a lot of help from the West.