You're missing the point. BB is WAR's highest potency combo. That means it can do its top DPS and still hold hate. Why? I dunno.. it just gets an arbitrary freebie there.
PS is DRK's lowest potency out of the jobs combos (PS 670 DE 680 DASE 800) which means it has to gimp its DPS to hold hate and if you stance dance and try to do your optimal rotation you're in very real danger of losing hate. PLD suffers from the same issue, but at least it situationally offers some utility with RoH. DRK additionally suffers since its enmity combo not only is its lowest DPS but destroys its mana-management.
I have almost every combo-based job leveled to the point where I have access to all their combos (PLD/DRK/WAR @ 60, DRG @ 60, MNK @ 50, NIN @ 46 (so I'm only missing Armor Crush)); I've pushed almost every combo in this game and its a fact: Power Slash isprobablythe worst combo in the entire game. Every DRK fucking hates it - lowest DPS, zero utility, AND it completely undermines the main mechanic of the job.
The potency of the two jobs' combos relative to each other is irrelevant. Its about the potency of the combo itself relative to the jobs' other combos in their rotation.