Considering theres like 18 ways for rogues to get sneak attack against undead, I don't see how this is a problem.
Theres a +1 enchantment that requires ghost touch as a prerequisite that lets them add their sneak attack. Same book, and it costs a little more.
There is a rogue class choice that trades trap sense(crap sense) for 1/2 sneak attack dice against non-crittable things.
There are spells that allow sneak attacking and critical hitting undead.
Also, the cleric in the party is still always going to be better at killing undead than the rogue ever will.
Also, I generally find that rogues are far from one of the strongest damage dealing classes in the game...All this does is make them less useless in a few encountes per adventure.
The bonuses the weapon crystals add seem to be cool, but comparing the ability to sneak attack undead to the other properties of a +10 weapon is silly to me....That's like saying it would be broken if you let a druid add the animal friendship property to his ring of elemental command. Say what?
Maybe I am too used to seeing fights where we encounter undead and anyone who doesent have power attack or turn undead just sits outside the room and waits for the fight to be over. I'd rather see a happy player whos spent his resources and can now contribute more to more types of encounters. Afterall, undead encounters certainly shouldn't be there JUST for the sake of denying sneak attack to your 1 lowly rogue.