I didn't like Ds9's take of the mirror universe, at all.
Sure the premise of a universe where everyone is evil and does the most horrific thing yet somehow, somehow, they manage to always survive and wiggle into roughly the same positions on ships and have the same kids/or be born at all is already a bit much to believe (the alternate of having *similar* actors in roughly the same roles being too expensive, of course, though) but then they have the Terran Empire collapse due to Spock's Reforms.
That sort of killed what the Mirror Universe was. Spock's reforms should had failed and or stagnated the empire much like how the Federation getting off war footing due to the end of the Klingon Cold War led to a sort of effete, if capable, Federation. Not being wiped out writ large by a Klingon-Cardassian Alliance because somehow, Spock, the minority ruled by the bloodthirsty Humans, manages to survive long enough, gather enough strength to, and manage the Human Empire who look at his kind barely better than slave animals...? And then the Empire collapses and the K-Cs take over.
And then somehow the Terran Rebellion makes a Defiant Class Ship with just Terok-Nor-ish. Huh? And sure the Terrans and Vulcans are slaves, that's bad. But what the hell is the rebellion going to end up doing? Already ISS is a imperial prefex...Smiley or Sisko might be vying to somehow make a run for the purple....
