The biggest problem with the mirror universe is that I don't care what happens there. It might be the same actors playing the same roles but in reality they are not the same roles, I know nothing about them and even when a main character is killed off it has no effect on me at all.
(Exceptions would be Lorca who we got to know in the Prime universe first, Georghiu who came into the prime universe, the Mirror Spock and some characters in DS9 who were nasty in the Prime universe but really nice in the Mirror universe - such as Brunt).
I felt the same with the JJ Abrahms reboot films.