Yes, it allows them to avoid a certain amount of damage but there is a limit to how much damage it will protect against.
If adaption was a simple 'IMMUNITY SWITCH' ... Star Trek First Contact would never happened. By the time the E-E got there the Borg would have adapted to Federation weapons and stormed right through, instead we see the cube is steadily worn down and a concentrated fire burst tears the thing apart.
Frankly, I think the Elite and Heavy Tactical drones need to have their immunity removed. It is extremely stupid to have drones that can survive 5 folks hammering away at them.
At best Adaption should offer an increased resistance against the damage type. Something like 50% capped with increased absorption from their shields so bleedthrough is reduced considerably. If anything, the power of the Borg is the fact their shields and weapons are so strong, not the insane concept Borg drones are walking tanks with huge HP.
As such, lower their health and increase their shield amounts. Offer adaption resistance to that shield with NO bleedthrough or extremely reduced bleedthrough. That way, one their shields drop the Borg will drop like they should. Apply the same concept to their ships and you have a suitable arrangement.
I agree with this. This type of arrangement is how I would see it working, and if you increase the difficulty that increases the HP of the shields and drone HP as well as improves their tactics.