DRG is the only heavy DD job I've used on FFXI, and although I am experienced with the game, I still am not entirely sure what separates it from the crowd (WAR SAM DRK MNK), so to speak. (In other words, why it's the weakest of the heavy dd)
Is it due to the power of the weapon skills? Ukko's Fury is pretty potent damage, Great Sword has some good weapon skills as well.
WAR and SAM get both Berserk and Hasso along with high multiattack rates (DA trait, Hassozanshins). MNK gets Impetus and Berserk along with the DoT advantages of a DW-style weapon, and decent TP gain to boot now that their delay per hand is down in the 180 range. DRK gets Berserk or a reduced x-hit along with their own Hasso-Berserk hybrid, plus the ability to use a stronger TP set when fully buffed, and potentially some amount of gain from Scarlet Delirium and/or Souleater. DRG gets Hasso plus... jumps and a pet, which are increasingly marginalized as buffs increase and further marginalized by the impracticalities of keeping a wyvern alive due to SE's cluelessness. Their WS aren't exceptional, but their power compared to other jobs depends on buffs and target. If other jobs can push higher pDIF values than DRG with their attack bonuses, it will suffer. If pDIF is somehow high enough to enable use of Drakesbane then DRG starts to catch up, but it still lacks in many ways.
DRG was built to be a moderate-output DD who could shed enmity to improve their output relative to other DDs who might have to hold back against stronger targets, along with some utility perks. In an era where everyone is loaded with buffs and caps enmity in under a minute, their perks have become irrelevant.