Even if we ignore the enmity changes - i.e. aggro combo skills removed - the answer is still clear as bright daylight.
1. DRK used to have another combo finisher - Delirium (the original one): you barely/never used Souleater as your combo finisher if it wasn't DA-buffed. Delirium had a higher potency than Souleater without DA (the debuff was just a bonus)
2. Scourge removed, and thus no dot-management: While many jobs lost their single-button-dot (NIN-Mutilate; WAR-Fracture [also cross-class]; MNK-Touch of Death) DRK probably got hurt the most because it used to be DRK highest potency skill as well as had an awesome animation. Salted Earth as the remaining "dot-management" just doesn't cut it. Neither does Living Shadow!
3. Dark Dance was scraped to Anticipation, then got removed: DRK "special" CD right now is Dark Mind, but it used to have a "physical opposite/sibling" - Dark Dance. While PLD lost Convalescence and Awareness, it was available to all tanks in HW, so ALL tanks lost those CDs. But Dark Dance got stipped off of DRK, contributed towards all tanks in a weaker form, then removed with 5.0. A whole CD lost without any compensation!
4. The increase of mana generation in SB led to the infamous "DA-Spam": DA in HW was fine; the constant MP drain from Darkside made mana management exciting and rewarding. However during SB, Darkside simply stopped your natural MP regen, you couldn't lose its buff anymore, and with the additional mana changes to Sython Strike, MP was overflowing. And the only or most effective way to spend that overflow was DA, DA, DA! A simple mana adjustment change would've been enough to decrease DA usage, but DRK was barely touched in SB, until the very end when they FINALLY buffed their enmity generation and defensive CDs. But nothing has been done to address that design flaw.
5. The destruction of DRK's identity in ShB (5.0): You may have noticed all those "Inner Delirium", "War 2.0" etc. jokes. The difference between Warrior and DRK Dps rotation/actions has become so thin, you can easily tell they are same with very little niches. i.e. (Blood) Delirium is the same as Inner Release, but instead of garanteed crit/direct hits, you gain little mana per hit; "Darkside" is essentially Storm's Eye on an oGCD (gain 30s 10% buff, max 60s); and Blood Weapon is an inferior Infuriate without charges, and yet you still need to get 5 GCDs into its window to "earn" that 50 gauge. No more DA - for better or worse. Also Bloodspiller is Fell Cleave! And don't get me started on the fact that DRK lost its Haste-based gameplay!Add that to the fact that WARs dps rotation is already dull without its stance dance, DRKs dps rotation is just as lame and boring.
The only action that was added to DRK - that wasn't just "press and profit" - is TBN. Tho, in my honest opinion, SE f*cked up here, too.
Firstly, it contradicts with the way, SE tends to design stuff. What I mean is, they introduced or changed actions to make them less punishing, or at least, they wouldn't result in a loss. i.e. SAM Third Eye doesn't cost anything, but grants you a slight dps (or health) gain when used correctly. TBN DOES cost! Namely, mana points. Don't get me wrong, the shield is strong and thus it prevents a lot of damage, it grants high survivability. But it also has a high risk - coupled with dps loss if it fails to break! And there aren't many situations or instances where it could be used (or abused) effectively. In fact, they are very scarce, sometimes even minuscule. (If you don't believe me, check out a certain website that records fights, and watch TBN usage count in optimised - thus "correct" gameplay. Last raid tier, the average usage count was about 1 every 2 minutes!) It being the very only action in the game with that kind of design/mechanic feels... out of place, and unfitting for FF14 (5.x). If it was easier to break, or you'ld have more opportunities to do so, it would be fine.
Yet that brings me to my second point: It used to be better!
First off, the changes that happened from 4.0 to 5.4. It started with a 20/10% shield on 5s (!) duration - in short, alomst unusable - 50 blood on break. The duration was soon buffed to 7s in 4.2. That were all changes to it during Stormblood. A fine action. In Shadowbringers however, the shield got buffed to 25%, in addition to the tank stance/trait changes, while the interaction with blood got removed. And for me, it was a huge downgrade from its previous iteration, because...
1. they removed hte interaction with the blood gauge, and changed it to "mana for mana" (3k MP for 3k MP worth "Dark Arts" - imo a perversion to its predecessor, just to keep using the term).
2. both, the increase of the shield and the passive 20% mitigation via trait, caused the situations in which the shield wouldn't break to skyrocket, and thus limiting usage of TBN in the "correct way" even further.
Right now, it's pretty much limited to (heavy hitting) tank busters, and big pulls in dungeons. There are still niche situations where it could save someones life, i.e. a tank that hasn't enough HP, but it would save healing and/or raising costs, or on a DPS/healer where a shield break is almost guaranteed in every raid aoe. (Tho, it does become obsolete when said DPS/healer doesn't have less HP than the rest of the party because it will be healed anyway.) It's pretty much on the same height as Clemency in those situations.
I think I've ranted enough.