Some time ago, summons were put down to level 1 but their stats (most of them) were changed to make up for the level loss. Undead summoned by Animate Dead do not seem to have been given the change in level, but have been given a turn resistance to make up for the change that didn't happen, or so it seems. My level 6 cleric of lathander (sun and good domains), even when rolling max on his turning, was only able to turn a basic Shambling Corpse summoned by an Impish Necromancer, not destroy it, while all the imps within range were killed.
I can only imagine how difficult the other, special, summoned undead would be to turn.