UndeadHalfOrc: Is the Undead Hunter worth it over the ordinary Paladin?
Immune to hold - don't care, there's tons of items and spells to prevent it
The real bonus is immune to Level Drain. Are there many monsters using this ability?
Assuming you are talking about BG2:
There are Vampires who drain 2 levels with each hit. You may encounter them in Chapter 3 (depending on who you side with; it should be obvious which choice has you fight vampires), and you will encounter them in Chapter 6.
In Throne of Bhaal, your level should be so high that level draining shouldn't be much of an issue, and with a Cleric or single/dual class Mage, you can remove it easily. (A Mage can cast Restoration via Wish or summon a Planetar with that spell.)
If you are using this class in a different IE game (either via mod or Enhanced Edition), note that those games didn't originally implement level draining, so I would expect level draining not to occur under typical circumstances. (It is worth noting that Icewind Dale does have a lot of undead; it just doesn't have any that drain levels.)
Also, note that, unlike in the SSI games, being level drained doesn't cause permanent experience loss. On the other hand, being drained to level 0 is a significant risk and will cause death (which, on your main character, ends the game immediately).
