Everything just has to lead back to Xeno doesn't it...
It's a pretty difficult argument to make, defending DRK, once you start comparing just how insane WAR is, it's such an unfair comparison that it's not even worth making in my opinion. That goes for the other tanks as well. WAR is just that busted in dungeon AoE self-healing, something almost 100% of the playerbase has to deal with, unlike something silly like a DPS check.
Honestly, it's much worse from a healer perspective. Unlike the other tanks, (WAR esp), Healers have an ACTIVE responsibility to assist the DRKs mitigate. This is why you see so many new SGEs be totally unable to heal DRKs, because they play it like a WHM or Eos-less SCH. I leveled all the healers to 90 recently, and got quite a few DRKs, both bad and good, and here's what I've noticed personally, a bit of a ramble, but whatever.
WHM
Wow, WHM's really did get the short straw this expansion, YIKES. The first thing I would do is tell the DRK in the party to use Living Dead. Most DRK's expressed genuine surprise and this, one even saying that he didn't even have the skill bound on his hotbar due to how trash it was. That should be a sign, Square Enix. 1st mob pack was ignoring all my responsibilities as a healer and going straight damage with PoM and Holy, forcing the DRK to LD or die. Wait out the timer, benediction as late as possible, then throw out the Asylum, Regen, Benesion, and if high enough level, Aquaveil to increase the strength of TBN and Benison. Solace as needed to generate Misery, Tetra early in case the pull drags on too long. Second trash packs, pull out Temperance, Thin Air + Medica II as we're walking up to the pack for huge regens, throw down Lilybell, and hopefully Misery as soon as possible.
DRK healing on WHM was never really a problem, even if I had to save Benediction for LD. As long as they used mitigation, and the DPS did comparable or higher damage than myself pulls would end far before they ran out of resources or I was forced to Cure II. A solid experience.
SCH
Now this is where it gets really good actually. SCH by FAR took the least amount of effort. Why? Because SCH has like, infinite tools. I have an answer for literally everything. Starting an pull? RecitExcog. That's the default, every pull will have one. By the time it wears off, you'll have a regular excog ready to go. Protraction at the start to make TBNs stronger. Sacred Soil at the start for regens, and to make TBN stronger. Fey Illum and Whispering Dawn off Eos for more regens and to give her something to do. Art of War is really great here, can weave in a lustrate or whatever oGCDs necessary. As the last enemy is dying, I usually dissipate to get those free aetherflow stacks, pray for a crit adlo as we are running to the next pack, and that's where Expedient, Fey Blessing/Union and Seraph would come in, making the 2nd pull much smoother than the first. After this point, there's an overabundance of Fairy gauge, so I found SCH exceptionally easy to steamroll dungeons on as long as I repeated this pattern. I would almost never have to resort to Aldo spam, and of course Physick remained firmly off my hotbar. Very enjoyable experience to heal DRKs with this one.
AST
Pretty similar to SCH really. AST's cooldowns are so short, every single pull I'd have a wide suite of oGCDs to keep the DRK alive. This is where I was punished the most for DRKs not using mitigation though, as I wouldn't be able to sustain a high amount of throughput without neglecting my other responsibilities without Lightspeed, like proper carding, or astrodyne set up. Couldn't pump out as much damage as WHM, but I gave DRKs melee cards on their two minute bursts and saw pretty agreeable results from it. I would request them to use LD on the 1st pull after every boss if they could, since Macrocosmos gets the most value there than anything these paper tigers of boss fights can do. I would start every pull incredibly strong, CU, CO, Intersections, I had a lot of options, but I feel like I ran dry a bit more often, possibly due to overextending on Essential Dignities, I'm not a very good AST. On AST I had to dip into Benefic II way more often, since I didn't have auto-healing like with SCH/SGE or stunning firepower of Holy spam WHM, with situations possibly devolving into a Synastry + Benefic II spam when DPS really couldn't be bothered to do damage, and myself as AST being unable to make up the difference outside of Gravity II with 2 sealed Astrodynes.
SGE
I'm pretty sure a good SGE and a good DRK combination is absolutely busted. You get the DRK damage, you get the SGE damage, and you get the sturdiest TBNs. With the massive amount of oGCD, self-reapplying shields SGE brings, things like Kerachole, Holos, and Taurochole combine with Oblation, all of those oGCDs having short recasts that further amplify Haima, Panhaima, and TBN. The only thing I would ever have to alternate between pulls were Panhaima and Haima, everything else was easily handled by these other incredibly powerful oGCDs, with me spamming AoE the entire time, applying Soteria once the Panhaima/Haima shields fell off. Even on first pull where I would use the weaker Panhaima, it was more than made up for by a Krasis+Physis II and Zoe+Pneuma combo once I let the DRK get a little bit low after I had used the initial suite of damage mitigation. It's not like I needed weave slots, or that any of these had particularly long cooldowns. SGE is a true maverick in this department, and I not only performed the best using it, I had the most fun working together with general tank mitigation for mutual benefit. Seriously, this comp is brutally good if both players are playing at their peaks, I cannot recommend it enough.
You know what the problem is? I had to use my entire kit on all healers to heal a DRK in a way I thought was comfortable for both sides. Since I'm of the mindset, "Perfect practice makes perfect" I would play dungeon pulls like I'd be in an extreme trial or something. Use your entire kit, ABC, space out mitigation properly, do as much damage as possible, eat food (since it's so easy to make now/EXP), etc etc etc. This was MANDATORY. The same thing was demanded of the DRK, even when I play it. Reprisal, AL, doing actual damage, the works. It was actually quite enjoyable and easy when I found the right tank.
But when I healed the other tanks? I still did all those things, but it didn't feel nearly as needed, and if the DPS were slacking, as long as the tank pressed even a handful of mitigations, I don't think I'd be any the wiser. The high skill floor for basic adequacy on dungeon DRKing is not conducive to players who are not willing to put forth the effort to not be at a bare minimum competent in their dungeons on each party slot. If you get a Cure Iing WHM, or a Diagnosis-only SGE, or a single-targeting anything, you as a DRK are going to die, painfully, there is nothing you can do about it. Just take the penalty, it will be a lot less annoying.
When I had good DRKs, DRKs that would play like I would in dungeons, it was easier to heal them than all of these bandwagoning WARs who think that 1 Bloodwhetting is the only CD they need to press in an entire dungeon pull. For the casual player who doesn't press all of their buttons, healing DRKs or tanking as a DRK must be an absolute nightmare, which would explain the discrepancy in experiences. I've never had any problems healing DRKs or being healed as DRK if both parties were capable of playing at even an acceptable level. But you need all parts of the team to be doing that to really get the most out of it in dungeons, while other tanks are running around soloing things all day, carrying people with literally nothing inside of their skulls, no problem.
So that once leaves us with the tried and true DRK problem that we are all familiar with. Noticeably more effort for the same overall result as other jobs in the role, now extended to the entire light party. I don't really care if they adjust this, since I like being challenged, I'm just pointing out what I've observed now that all my tanks and healers are at 90. BiS DRK is going to shred Smileton and Sigma pulls to pieces though. With damage CDs, I'm already slicing some packs down in less than twenty-five seconds with a decent group, the damage is already pretty obscene with a i600 greatsword. I'm not justifying this, I'm just saying Living Shadow and two Shadowbringers/SaD/Floods are enhanced multi-enemy deletion devices.