Not sure what you mean by "since the cross class purge they seem under equipped" they have access to the same skills they use to have, unless you mean the battle system revamp as a whole, in which case yes they "suck".
You are correct that all you are doing is spamming that one combo, and while its a good combo it gets pretty boring, since there is nothing to do otherwise between sets. Plus before I get into the Blackblood issue, I just wanted to say that SE removing Scourge was a mistake. I am aware that they got rid of pure DoTs as a whole for most Jobs, but DRK was the only tank that their DoT worked in their rotation. People said they kinda want a DoT tank and this was SE's chance to do that similar to Bard.
Anyways the main issue I have/see with DRK is the resources they have to manage. Removing Delirium, or a second combo finisher rather, was dumb, they just should of changed the effect of Delirium; why not have it buff their magic damage; or better yet put Scourge in its place. Why not remove/revamp some of the fluff they have with spells like Dark Passenger and Abyssal Drain. While they are decent skills, their MP cost combined with the no MP regeneration, PERIOD, makes them not too user friendly. I'm sure in any big pull Unleash and Salted Earth will be the main source of AoE, as it maintains enmity while dealing decent damage. Besides the new go to for AoE is Quietus since it restores MP too. Why was Blood Price made to only work when you have Grit active, it's an MP restoration skill, you use the most MP when you are using Darkside, even without Grit active. The change to not being able to regenerate MP at all was "different", but not too terrible, adding Blackblood, not terrible, both together, they are terrible.
BlackBlood is one of the most annoying gauges to well gauge. It only charges with one attack and 2 oGCD skills, very slowly. Compared to the other two tanks whose gauges charge at a reasonable pace. For WAR, to use one of its gauge skills you'd only need to utilize 3CGD skills for the 20 cost skills, and 6CGD skills for the 50 cost skills, or just use the instant 50 oGCD skill. For PLD, there Sword Oath gauge increases by 5 for every auto-attack which is roughly a GCD on its own, in this regard you'll reach 50 in roughly 10GCDs. Sheild Oath is even better, because while the increase is tied to their blocking mechanic, their Holy Spirit spell increases the gauge by 20 while in Shield Oath, thats 3GCDs to use a gauge skill. But DRK, oh DRK...
DRKs gauge increases only with its Souleater skill, which requires 3GCDs to reach, and the gauge only increases by 10 at a time. since the minimum skills need is 50 to use, that means we have to wait 15GCDs to use just one of these skills. Granted PLD while in Sword Oath needs roughly 10GCDs to use a skill, but during that wait time they, have other stuff to do. DRK only has the one combo, so you are literally standing there spamming it, even at max level.
I can't say what I would of preferred to see when it came for the new DRK to be revealed, but I wasn't expecting this. DRK was my primary tank in HW, but now its a sad pale imitation. And the main reason in my opinion is the disconnect between its gauge and its skills; plus the lack of another branching combo.