The way I see it, Sima Yi had a harder time in Wei than that of Zhuge Liang in Shu. When Liu Bei die, he made his son to treat Zhuge Liang as a father figure, this mean that many people in Shu could disagree with Zhuge Liang, but they could not undermind his authority. Sima Yi was not that lucky, even when he was given high rank, his had to thread very carefully to not allowed his political rivals to destroy him. Not only that, he had to defend two fronts from a two fierce rivals in Wu and Shu.
Most people excuse Zhuge Liang failure in his northern campaigns due to internal politics at home and a weak ruler, then how about Sima Yi? Cao Hui was not what one could call an ambitious ruler, and that Sima Yi never had any real power during the time of Cao Hiu, the Xiahou clan rely on the long service of their ancestor always wanted more power in Wei and usually oppose to whatever Sima Yi do.
That basically my thought about Sima. Any other criticism?
