Ranger REG
Explorer
Compared to the 1e/2e Thief's Backstabbing ability, Sneak Attack is a lot more useful. I don't mind if the undead is immune to critical hits. They don't have living vital organs that would make them double over in pain. As for the Improved Uncanny Dodge vs. Sneak Attack, you can always surprise and still inflict Sneak Attack damage. Otherwise, find a much weaker opponent to finish off (usually enemy spellcasters are the main priority).