Uncanny dodge let you keep your dex bonus to ac when fighting an invisible opponent. OK his my question. It is only the dex modifier part that matters. Flanking is a second option. For that, the Uncanny dodge description is quite clear and has been pointed out numerous times already.
Uncanny dodge makes no mention of avoiding the affects of a feignt.
A successful feignt makes the target lose their dex modifier, therefore they become a valid target for a sneak attack. OK his my question. The rogue or barbarian would not be denied their dex bonus, even though other classes would. Higher level rogues and barbarians can still be sneak attacked, it's just more difficult. Uncanny dodge just prevents you from losing you dex bonus when you are flat footed, fighting an invisible opponent, and it protects you from being flanked.