Currently most of pierce ranged weapons can’t reach 100% conversion and have to rely on one belt to have a respectable level of conversion of physical to pierce (Mythical Pack of Treacherous Means). Meanwhile piercing swords never use Blessed Whetstone because they have a specific devotion that coupled with most piercing swords and Belgothian set enables 100% physical to pierce conversion.
I can’t see a balancing issue here, but at least ranged pierce builds will get some life and diversity and Blessed Whetstone will be good for something.