Counterattack gives you 50% chance to respond to attacks of adjacent to you enemies with 55 power attack. However, unlike almost every other reactive damaging ability (-riposte) it relies on agility. It also can be parried or blocked, procing heavy guard or riposte in the process. So you can get hit twice by 1 attack just by simply having that ability. Admittedly, heavy guard is a very rare ability, but one particularly pesky hero can do that blocking/parrying/slamming vombo combo to you if you happen to have counterattack. It comes in a pack with sure slice as a staring ability of titan's fist, which sort of promotes non-agility type of build.
I suggest make it not reliant on agility and reduce damage from 55 to 40 power with chance of procing staying the same, just like dark/light veils.