We sometimes receive tickets about the Rocket Silo failing to fire on enemy units during defensive battles. In the majority of cases, players refer to the Achilles GS as the rocket that fails. The Achilles and Achilles GS rockets are actually working correctly; in most cases the issue is just a bit of confusion surrounding the different targeting rules for those two rocket types.
The Achilles rocket targets LIMITED units, including all Special Forces, Hero units and all Unique units. Any unit you cannot build more of than you have unlocked in the past is considered a Limited Unit.
The Achilles GS rocket targets unique units. This only includes units which are true unique units; you can tell these units from others by mousing over the unit in the research or production buildings. When you do so, True Unique units will have a Diamond Icon like this one:
What about the Herald?:
The Herald is a true unique unit, However, only Pure AA units can target it, because it is also a High Altitude Air Unit. Since the Achilles GS will also target ground based-unique units like the Juggernaut or War Rig, it will not target the Herald. High Altitude units are signified by this icon:
What about Heroes?
Hero units are limited, but they are not true unique units. The hero designation will be represented in the game with this icon: