Realistically speaking, war hammers were used specifically for breaking through armor:
War hammers were developed as a consequence of the prevalence of surface-hardened steel surfacing of wrought iron armors of the late medieval battlefields during the fourteenth and fifteenth centuries.
In Dungeon World there is the Piercing weapon tag, it means specifically that:
n Piercing: It goes right through armor. When you deal damage with n piercing, you subtract n from the enemy’s armor for that attack.
However, neither Warhammer nor Mace have Piercing. They have no difference from swords and axes, actually:
Short Sword, Axe, Warhammer, Mace; close, 8 coins, 1 weight
As a DM, I want to add 1-2 Piercing to war hammers in my game. My concern is, there is only one weapon that actually has Piercing in the default equipment (a "dueling rapier", whatever it is). In terms of game balance, does adding Piercing tag to warhammers break anything, forcing GM to justify consequences accordingly (for instance, make warkahhers heavy and harder to wield), presumably adding more tags (like Awkward)?
By "balance" I mean that game should feel fair, and nobody steals the spotlight from any PC.
Sister question: Does adding the 'precise' tag to daggers break anything?