The identity of the DRK as a "Magic Tank" is dead.
Its a failed concept as it is practiced in FFXIV. I think it could be cool but as it stands its the same thinking as 3.x with PLD being the tank that takes physical damage. The only saving grace for PLD was the majority of TBs and damaging attacks were/are magic based so SE had to right that wrong faster.
I wanted to specifically address Heart of Light (GNB) verses Dark Missionary (DRK) AOE raidwide shields as an example of this. Disclaimer: Maybe the tooltips I've found are incorrect. If that is the case the please if someone would link the accurate versions so I can avoid communicating bad info. As it stands I found this from a reputable source so I'm running with it. Heart of Light: Flat 15% damage reduction, duration 15sec., recast 90sec.. Dark Missionary: 15% magic damage reduction, duration 15sec., recast 90sec.. The radius of effect is the same for both. The result of being a "magic tank" is DRK being forced into the box of only blocking magic (not even being something it is good at when you compare kit of other tanks). The step towards homogenization is real. While I'm frustrated with that, it's not a deal breaker for me. Being that homogenization has occurred why not make theses both flat 15% damage reduction shields - "Magic Tank", thats why.
"Magic Tank" was an okay theme back in HW because it was piggybacked by other mechanics in the game like evasion tank and parry tank but that was then and those underylying themes are gone; Magic tank as a theme should be gone too. IF the dev team said DRK would excell at taking magic damage in addition to a bevy of on par CDs I could be okay with that (as other tanks should excell in different areas) but this isn't the case. Instead what we get is a model of DRKs that take magic damage at the level everyone else does and is subpar at everything else. And that is the heart of why "Magic Tank" has failed. Make Dark Missionary TBN-like that gives health/mp back or a flat damage resistance buff on par with heart of stone so it can be utilized as a personal CD for extra mitigation time allowing. Oh, why nerf Dark Mind? Fix that too or rework it to get away from "Magic Tank".
Magic tank is cool in theory but SE has failed to make it work. It's simply gives devs a reason to say that almost everything mechanically is magic damage focused therefore, since they are the "magic tank", we can make their overall kit subpar because DRKs, by the very nature of their job, benefit from game mechanics. This is why no rework has succeeded to resonate with the community, the underlying premise of the job is flawed so everything flowing from that will by flawed as well.