Cao Cao's biggest failure was not uniting the lands of Han China by the time he died. He tried, but failed. He attempted to go through Jing and Yang and was close to mounting an attack on Yi. In the end though his failure led to 60 years of nearly constant warfare until China was reunited. Time that eventually allowed his second biggest mistake (bringing the xiongnu into the interior) to grow the point where it destroyed "Han Chinese" rule for several hundred years. Keep in mind that he had from 208 until what we will assume would always be his death in 220. That's 12 years. In that time he launched his infamous Han Zhong and Chi Bi campaigns, two assaults on Ru Xu and an expidition into Liangzhou.
Anyways, do you think that Cao Cao failed to united China becuase he made his own mistakes or was it a lack of time? Did his mistakes create a position for his enemies that no attack would be able to root them out? Or would he have defeated them with enough time?