The Pillbox is a simple, manned bunker armed with a heavy machine gun. Though it is effective at eliminating infantry and very lightly armored vehicles, it lacks the firepower to easily destroy heavily armored units.
Pillboxes function as the first line of defense against infantry. Its inexpensive cost and moderate hit points make the Pillbox fare well against infantry and light vehicle attacks and a deterrence to even the most formidable infantry if they are easily killable and/or unable to harm defenses (like Engineers). Their lack of power requirement and quick deployment also enables them to become a diversion and a make do defense in under extreme situations (such as sudden subterranean attacks from Driller APCs).
The Pillbox is not designed to tackle heavy armored vehicles and siege vehicles, so it must be paired with other stationary emplacements to increase the chances of a successful defense.