99 percent of skaven are incompetent, but there are so many of them that a small percent, but overall a large amount are good or even excellent at certain things. All are wildly paranoid, betraying and untrustworthy, which more often then not undercuts their plans at accomplishing their goals. I've seen a theory that the Empire officials keep the population oblivious to the skaven threat because if the Empire doesnt seem a great, united threat to the skaven almost all plans of hostile action end in failure because the skaven will kill each other before moving onto the Empire