I mentioned the moral aspect because the only way the Alpha quadrant powers can effectively fight an enemy as massive as the Borg is to unite their forces.
Since the Klingon-Cardassian alliance is founded on the basis of brutality to other races,those same races won't lift a finger to save it against the Borg.
Let's say the Alliance wakes up and asks for help to stop the Borg.The other races may actually stand by to allow the Borg to do their dirty work for them by eliminating the Alliance for either revenge or 'the greater good' depending on which species you ask.
At least they'll see it that way until the Borg show up in their backyards,by which point it'll be too late.
Such a nastier, morally bankrupt civilization founded on the basis of brutality might have in their arsenal even deadlier, more devastating weapons than the morally pure civilization founded on the basis of universal love and harmony we've all come to feel warm and fuzzy about. Thus the morally bankrupt civilization might be better off in taking the Borg on head-to-head.
One wonders what the Q or Organians (to name just two) are like in this universe. Also of note is that not all species were mirror opposites. The Halkans were puny wimps in both universes.