Gun! Gun! Gun!

[The Coil]

I've always found the 'tank fist' concept most helpful on defense, where you can move it around under cover and lie in wait, in the manner of your 'line of death' pic, POS. It's a great way to get maximum firepower on the opponents AFVs as the come in to sight one at a time, especially as your vehicles are generally stationary, and acquire targets quicker.

For me, always been too risky on the attack, especially against a good opponent. 12 tanks on 1 gun, you'll probably be OK. But drive that fist into LOS of 3 guns against an opponent who knows when to fire (in the manner of the second pic), and your fist will be hurting. As a defender, I'm usually very happy if I can trade a gun for 2 tanks. An experienced gun crew will get that on a tank fist before it's even spotted.

The advantage of the tank fist on the attack, on the other hand, is that your throw your whole weight at one small slice of your opponent's line, minimizing the amount of troops you are fighting with the fist, unless he's done a good job shifting forces...