Well Knives generally have two weaknesses

Because they split their attacks into two they essentially double the defense of the opponent. So if the opponent reduces damage by 20 points, they reduce the entire volley by 40 points. They have an impossible time against high defense opponents... or at least they should.

The other is that if you aren't getting a back attack, it is a wasted attack. While quite a few enemies prevent you from going there.

To balance knives just emphasize the above. Create more of a need for maneuvering with rogues.