Considering that most dragons have massive information nets just getting past their security is going to be a trick. The Dragons themselves are ,as other people have said, tough. The military attacked Alamais with multiple vehical mounted lazar cannons. They thought he was dead and then he turned up again. Killing the Dunkelzahn was a major event and unlikely to be pulled off by petty runners, grade A runners or anything short of a megacorp. Aztechnology had all those toys and Ghostwalker crushed them in Colorado. Hetsaby drove back the military forces in the war between CFS and TT.
Even if you did kill one great dragon, do you not think the rest would start looking for revenge and how many runners could withstand the full influence of MCT and SK being turned on them. That is just 2 dragons.