Dawn of War Wiki

Tankbustas are the GODS of Ork infantry. Their weapons are effective (or at least excessively damaging) against both vehicles and structures. As for infantry, they are less effective against both regular and heavy, but can disrupt and knock down a squad with their barrage ability to pave the way for more suited Ork Boyz.

Back on the topic of their main uses, Tankbustas will pretty much tear apart any tank. A single squad can heavily damage a vehicle, and outright destroy it if it is caught unawares, especially if the attack is lead with a barrage.

Two squads can easily cause a single vehicle to turn tail, usually destroying it in the process.

Three squads? Well, if you really, REALLy don't want your enemy to have a single vehicle on the battlefiled, 3 squads of Tankbustas will get you your wish. 3 squads will halt a vehicular assault almost instantaneously, and again, using a barrage to lead your attack will only capitalize on this. Once your enemy completely lacks vehicles, you can then spearhead your way into enemy territory and annihilate your enemy's base with those tankbustas. You don't even necessarily need vehicles; a barrage and 1-2 salvos is enough to destroy a turret and leave a flank vulnerable to MORE ROKKITS.

Any more than three squads is just overkill, and you will usually be left vulnerable due to overspecializing. But, you will end any vehicle/buildling incredibly quickly with them.

TL;DR Tankbustas are freaking awesome against any vehicles, or in a base assault. They can also be used to supplement a force against infantry. No Ork army should be without them.