I don't think the Kor'kron by itself was all that strong, the problem was Garrosh was into some bad stuff and needed removed. Yeah it took a lot effort to get into Org but like in real life few people can hold out in a well fortified position when assaulted by many.
I would say that the SoO Kor'kron where less powerful as a military then the current army of the scourge under Bolavar. The Kor'kron wouldn't have been more then a bug to squash in the path of the Alliance army in combat in the open field considering a unified Horde was a match for the Alliance.
TL;DR - Orc's alone are weak. Warcraft history has shown cooperation is the only way to have victory.