Thats wrong. Tank destroyers are in cities the best units against armoured units. They can also make an air attack (level 3), together with airmobile infantry is the tank destroyer (level 3) very good in countrys with much jungle or mountains.
Tank destroyers level 4 with mech. infantry european doctrin level 2 are also very fast on the ground (speed 1,4), or tank destroyers (level 4) with marines western doctrin level 2 (speed 1,4).
The tank destroyer is the best unit against aa tanks in cities. But dont make an air attack against aa tanks.
In this game you must also look on the terrain bonus, not only on the attack and defense power and HPs.

Must concur with Seele - if you look at the "bang for the buck" ratio TD's bag the other units. Especially if you are playing on a new map with the latest balancing.
This being said, most of our units are situational - you can use them anywhere of course, but if you want to optimize their damage you need to look at the tactical situation and that means terrain fist and foremost.

//G

it's easy to set up a very Stalingrad-ish style of defense in early game thanks to the tank destroyer.

Put a bunker lvl 1 (quick to build) with 1 motorised inf and 1 national guard (for the buffer) + 1 TD, and you will force your ennemy to use artillery or air attacks. Nearly anything by ground will fail miserably. For Euro Doctrine it basically deals 15 hard damage when in city defense. It can wreck several combat recon vehicle accompanied by infantry, before showing signs of weakness.

I won't go further on what Selee explained on their airborn attribute, and the fact it's still a very good specialised unit, but more on the defensive early game aspect : they are the most efficient unit at creating a "damage gap" at a period of the game where entrenchment and difference of values between off and def (for infantry) settles most of the fight. Basically, in early game, it's the counter of the counter of the starting-inf armies.

1 motorised + 1 national guard + 1 TD in a lvl 1 bunker

30 soft HP + 24 Hard HP. A good balanced ratio between hard and soft, that will help mitigate some damages from the specialised units (LOOKING AT YOU COMBAT RECON).

In terms of damage, you have

soft : (4.5 * 1.25) + (2.3 * 1.25) + (2*1.5) = 11.5 soft damage
hard : (3*1.25) + (1.5 *1.25) + (9.5 * 1.5 lol) = 20 hard damage

And you add to the top of it the entrenchment + bunker bonus giving a 45% reduction damage.

In this situation, and against this composition, if you are rushed by a group of

4 infantries (60 HP) and 3 combat recon vehicles (75 HP) dealing :

Soft : 6.6 + 9 = 15.6 soft damage
Hard : 4.4 + 3.3 = 7.7 hard damage

You.will.win. The low cost national guard sacrificing itself to buffer long enough for the TD to crush the first recon, reducing drastically the soft killing power.

The beauty of CoN being : of course, this is not the ultimate defense. It can be crushed easily by gunships and attack helicopters. Or missiles, or strike. Or artilleries. All defenses are situational. But here, if you manage to establish this kind of defense against ground-rushing countries in early, i can assure you that the TD will virtually allow you to withstand one country laughing, two countries with relative security, and three countries with a bit of luck.