I was the one who sent the e-mail to the sage about the assassin, and he confirmed that it's supposed to have exactly the same sneak attack ability as a rogue.
Also check the DMG Errata. It updates the blackguard sneak attack information, and the assassin and shadowdancer Uncanny Dodge information.
Yeah, also the Uncanny Dodge is slightly off, I think, in both the Assassin and Blackguard writeups. They should be the same as the Rogue has. (One would think that copy and paste was a well established practice by now, but )
This WAS corrected in the DMG errata. They are the same as the rogue's. All sneak attacks work the same. Look at all the classes in S&S. It's just a mistake they missed.