Note that unchained rogue doesn't technically stack with sneak attack from any other class. Only unchained gets to ignore concealment. Other classes have the sneak attack from regular rogues. In practice, they generally stack though. I took the shadow strike feat just so I wouldn't have to deal with the issue.
The knife master archetype for rogue though, as well as anything else that modifies sneak attack, does apply to other classes, as there's nothing that cares about where your sneak attack comes from.