They were also thought "incompetent" before flying planes they could never learn to fly, somehow taking out three buildings and 3000 people, including four commercial jets full of people. The coordination and planning of the events were amazingly effective. All the fanatics need is one or more of the Russian's "lost" suitcase bombs, some OBL cash, a coyote-lead free entrance across the Southern border and LA or Phoenix could disappear.
From what I have read they were not competent to fly aircraft. However, they were able to keep them in the air long enough to hit the towers, which I guess is what matters. Of course, I think it's irrelvent. I'm not saying the terrorists are devoid of any intelligence. Culturally, Islamic Arab terrorists are the most militarily icompetent people on the face of the planet. This is why they resort to killing innocent civilians through suicide attacks. Their nihilstic approach to war relies far too much on divine intervention and self delusion to be able to do something as complicated as smuggle and detonate nukes in this country.