My main is currently a level 10 Cleric, and has 3 Cleric past lives and Turn-boosting gear & enhancements, and so has a fairly effective Turn Undead.
However, I've noticed a bug with Turn Undead: when I have a very large crowd on undead around me, and use Turn Undead, it works, and destroys lots of them; however, any that remain become effectively immune to future uses of Turn Undead. If the first Turn destroyed half of them, great, but a 2nd Turn will do absolutely nothing at all.
If I hard-target a single undead, Turn will work on it. And Turn will still work on any undead that were not in range of previous uses.
But it's really annoying when 2 uses of Turn ought to be able to wipe out a bunch of undead, but after I got half of them with the first, the rest are untouchable.
I did bug-report this, too.