I see no flaw in OHK's, if you go up against someone wielding a massive weapon that takes TWO FREAKING HANDS just to swing...
Expect to get your shield splintered and your wig split.
Unless you could've gotten out of the way or hit them before they hit you. The way it is now, all the enemy has to do is decide they want to attack, and if that attack would hypothetically kill you at that instant, the game can randomly decide to insta-kill you without warning. It doesn't give challenge, it gives frustration. It doesn't matter how long the attack would take, and doesn't matter what you are doing or what you would do during the "wind up". Case in point: I watched someone play and they were getting low on health, when the camera popped into third-person the moment they went into their inventory. They healed up to full, and upon leaving the inventory, they could not move and got killed. Even though they tried to get out of the way and were at full health when the attack hit, the game already decided the player was going to die well before they actually did, and there was nothing the player could do.
The same thing happens with one-handed weapons, but two-handed weapons make the effect more pronouced as it does a lot more dmage and is normally a lot slower... but for kill moves the attack speed may as well be faster than light.