I'm not sure if there are items that only he can drop...

But take Griswold's Redemption for example... Frozenstein, Pindleskin, Thresh Socket and Snapchip Shatter all have a .0001% base chance of dropping it in hell. Then there are a few other act 5 hell monsters who have a 1/100th that chance...

There are a number of items that have similar drop stats.

I'm not sure exactly how the drops to work. But I know that some treasure classes can't be dropped by Baal, even though he is so high-level. I just use D2drops as reference ;) .

And I seem to recall that, though Nihl can't drop everything, he does drop the highest-level charms. That is, if you get charms from him, they'll either be good or rerollable to be something good...