It is not as good as it might seem at first look. Problems for Pox in general is chalice with one counter on the first turn. Then the eldrazi player can get a
Thought-knot Seer into play already on turn two, pinching your answer to it. Then
Reality Smasher can turn up turn three to help do nine damage. At that point it hurt too much to play toxic deluge.
Another (important) advantage of Dismember, over Innocent Blood, is that as an instant it can be cast in response to the Thought-Knot Seer trigger.