Ablative armor works by absorbing the distructive energy from the incoming weapon and vapourizing itself to reduce the damage (ST: Encypd). Kind of like the modern explosive reactive armor, AA will wear itself out if hit in the same spot repeatedly. AA is also not very effective again projectile weapons, especially delay-fused ones, like the Breen torpedoes during the Dominion War. They'll just punch holes in the armor and explode inside the target.

The armor in "Endgame" is tricky, since they didn't say anything about it. I think it refracs or reflects an incoming weapon's energy, which would also explain why tractor beams couldn't lock on. The graviton particles were being bounced off.

I think polarizing the hull is meant to put a charge on it. If the incoming weapon contains a charge, the same charge, the law of magnetism would mean at least some of it will be reflected off. This kind of armoring would work best against things like particle beams and ion cannons, anything with involving charged particles, but it probably next to useless against phasers, disruptors, and fused projectile weapons.