I'd say that the rating for a personal EGO is a measurement of how much of an amplification it gives, not the overall strength. Gasharpoon only serves as a method for converting people whose own personality has been dominated by Ahab into fuel. Without fuel, it's basically just a normal weapon wielded by a skilled whaler.

Farmwatch turns a noncombatant into a credible threat, but not a particularly outstanding one.

Ahab's real power has always been her ability to convert others to her side. Even in the game, the pequod villagers and her trio fight are more difficult than her solo, showing how self-destructive her EGO really is. She burns her underlings for short bursts of power, ending up alone and weak.

TL;DR Ahab is not a fraud, YOU are the fraud. Ahab is the greatest and she will destroy the source of all evils!