Crushing blow is not at all OP. It's just not.
While it may not be overpowered when viewed holistically (though I have my doubts), what I actually said was "Crushing Blow, in general, seems overpowered to me compared to the [other] weapon abilities (due, largely, I think, to the way attacks and defenses are compared when calculating damage; e.g., a single 20-point attack is much better than two 10-point attacks, especially since it also serves to immediately reduce incoming damage)."
ED: Added "other" back in, because apparently I forgot to write it.
Or, in other words, Crushing Blow is, hands down, the best weapon ability because a 20-point attack is better than 2 10-point attacks, especially when those 2 10-point attacks go against different units (which makes it better than Impale, and it is hard to get the enemy to line up nicely for a 3-hit Cleave, so it is usually better than Cleave, too). Yes, opposing forces might get one or two additional attacks in against the unit which used Crushing Blow due to the 1-round stun and the initiative penalty, but the effect of Crushing Blow's is to reduce enemy stacks much more efficiently than through normal attacks.
This is for the same reason that focus firing is the optimal combat solution - e.g., 1 full-health and 1 dead enemy is a far better situation than 2 half-health enemies in the vast majority of cases.
Yes, it allows weaker units to punch above their weight class. But the crushing blow weapons come with a significant initiative penalty. So you're almost guaranteed not to get first strike and if the target survives (or dodges!), your unit is likely to be swarmed and destroyed before it can move again. That's the tradeoff.
The tradeoff is not, at this point and of course in my opinion, equivalent to the pay-off. You are, defensively speaking, no worse off standing still for your 1-turn stun than you would be had you just stood still (or relatively still) and attacked twice, and, offensively, you are far better off than you otherwise would have been (because 20x1 >> 10x2).
If you can't take a town because it's defenders are using Crushing Blow, your army is really weak and you shouldn't be attacking that town!
Not what I'm saying. I'm saying that, when attacking a town, the enemy that is the most dangerous to your own army is not the Gigantic Animated Stone Statue or the ranks of archers;* it's the 3 or 4-member Level 1, unarmored militia squad with their 14x3 or 4 base attack and Crushing Blow, and they are especially deadly to melee-flavored sovereigns and champions (who would gladly go toe-to-toe with the Gigantic Animated Stone Statue and win handily).
As an aside, the game should display the name of the applied skill over the head of the unit that is using it. The current visual display of the AI using impale or cleave (where extra units take damage) is just bizarre. And as Fallenchar points out, the animation for Crushing Blow is not exactly obvious either.
Agreed on that one.
One other nitpick: I'd like to pick what items get upgraded when I upgrade a unit's armor. On my Ice Witches and Fire Warlocks (both warg-mounted), I want them to upgrade their armor, but still keep wearing the Soldier's Boots for +1 Initiative. Unfortunately, upgrading their armor replaces their boots along with everything else, even when the boots aren't set to "Upgradeable Boots" in the unit design screen. Additionally, it doesn't let me pick which cloak they change into - defaulting, I believe, to the anti-Fire cloak (fine for the Warlocks, disflavorful for the Witches).
* Actually, archers are usually pretty meh, too, because of the even-bigger-than-for-crushing-blow-weapons Initiative penalty and the lackluster base damage and the inability to use something like Crushing Blow to gain a quick piece-count advantage.