Although the Black Mirror is powerful, it has its weaknesses. The dark incarnations within the Black Mirror are all transformations of the darkness from their "kill three corpses" act. Ultimately, they are not their complete selves, so they are innately deficient and cannot be mentioned in the same breath as their true forms.
What's more, over the years, the true forms have been advancing at full speed, while the dark incarnations need time to grow into the strength at the time they were severed. This creates a gap in strength, which is further widened by the innate differences.
The longer the time, the greater the disparity.
The truly terrifying aspect of the Black Mirror is that the dark incarnations can be reborn an unlimited number of times! As long as the Black Mirror remains unbroken, given time, no matter how many times the dark incarnations die, they can be reborn with sufficient time."
