I know it's old, but I just ran Touch of the Abyss and thought I'd see if other DMs had a similar experience to mine. One of the players, after the game, commented that I must have been reading "Ability Damage Monthly" before the game!
The party's cleric (Jacinta, Clr11 of Pelor) specialised in turning undead and typically made light work of the first set of greater shadows, but not before the fighter (Eadwyn, Ftr8/Weaponmaster3) lost two points of Strength, on top of the Dexterity he lost to the reekmurk. The cleric spent much of that reekmurk fight throwing up.
The hezrou fight... is that thing really only CR11? Long story short, when my players encountered it, they were completely borked over:
When they finally beat him they heal up (lost about half their hit points each in that fight) and make it to Marquis, the party loses more Strength and the cleric is dropped to 2 Strength by the shadows before she can destroy them. The Marquis fills the corridor with cloudkill - Constitution damage each round - before Evard's black tentacles holds them in it. The cleric, at 2 Strength, needs a natural 20 to break the grapple of the tentacles, taking more Constitution damage all the while, and meanwhile the Marquis is having fun with enervation.
I think ability score damage was doled out more often than real damage! Was the author on an ability score damage kick at the time, or have I just not noticed how much of it there is at level 11?