Sherman is a horrible personnel guy. Horrible drafts, bad free agencies.
I think the thing with Sherman is similar to the knocks on guys like Dungy, Dennis Green, Mary Schottenheimer, and a bunch of other guys. He beat up on crappy teams during the regular season, and then the playoffs come along, when coaching ability is at a premium because the talent levels are much better, and they get blown out and/or embarassed and disappear.
I'm sure that Sherman is a prepared guy and generally has his guys ready to play on Sundays, but he made bad personnel decisions when he was GM, left the Packers' cap in a shambles, and was routinely blown out in the playoffs when not coaching aginst the Steve Mariuccis of the NFL.