But then again you leave yourself open to being hit, while you wait on the turn in which to spend all your defenses and reactions. Not playing any defenses could be devastating, as hits can be quite lethal in WFRP, so then the player is potentially taking a huge risk to make the combo. While at the same time the player risks getting hit even if playing all their defenses at once, and that would ruin the combo. If it pays off then well, congratulations you risked and you were rewarded (this time). Maybe next time the player won't be so lucky.
Also, I never let the players know exactly how large the creatures ACE-pools are, or how much I have left. Generally I hold on to at least some of the pools and use it at an opportune moment. I try only to use a trickle of A-dice, and then use A+E-dice when it really matters.
Furthermore, the combo would be quite useless against many low powered enemies (such as goblins etc.) as they potentially die from the first counter attack, making the other defense cards "useless" from a counter attack perspective. Also, one dead goblin from that combo might not be too much of a loss from the goblins point of view if there are a more of them waiting to attack (the now active defenseless player).
Also, the counter attacks doesn't do that much damage if I remember correctly. They are quite similar in damage as the Melee strike are they not? More powerful foes might easily withstand all the counter attacks. For example, an Orc in armor could probably soak 6-8 damage or so (I don't have their stats at hand), while a fighter with a hand weapon (the fighter probably has a hand weapon, since they need the shield to be allowed to block, which is part of the combo) might deal 8-12 damage, depending on STR-value and number of successes. So when using the combo against a more powerful foe they might only do a couple of damage on each counter attack, provided that the player succeeds. There's a good probability that the orc in the example survives and can attack the player brutally (since he/she has no active defenses left after the combo). Even worse, what if the Orc manages to score a hit, despite all the defenses?
So I can't really feel that this is an issue. The player using this combo will risk a lot and it should pay off but sometimes it might just go horribly wrong.