Is GNB bad or is TBN too good?

[Cosaint]

If we're talking about aoe pulls, he's not wrong though. I struggle to recall many aoe groups that have any relevant amount of magical damage in it, effectively making two of our cooldowns pointless in these situations.

Mostly everything with elemental damage is magical, like an Aero, a Fire Breath or Water Attack. While physical attacks are usually punches, scratches, bites, slashes, thrusts... You get the idea.
But then, what else PLD has? They lost Bulwark and Divine Veil doesn't work on yourself. Clemency can save your life, but it comes at a cost of a GCD and potentially a Holy Circle. PoA only works from the front and you need to stop attacking, I wouldn't even call it a viable cooldown. The only real edge is Hallowed Ground, which doesn't come so often. If you are paired with a bad healer, PLD can make it work with severe DPS cost, but it's not really the tank's job to play for two.

[Lersayil]

If you are paired with a bad healer, PLD can make it work with severe DPS cost, but it's not really the tank's job to play for two.

This last part is is basically the difference. PLD can smooth things out / save himself mostly on demand if the situation requires it, paying for it with dps. DRK baseline is slightly better (albeit a somewhat wonky due to the day and night difference between the TBN and non-TBN moments), but pays for it with having only one sustain skill on a large cd, which you have to save and not burn for dps if you want it for the heal (also the 1 less mitigation cd if we're not counting the minor AOE one).

Overall I would need actual numbers to make sweeping statements, but its a trade-off. A trade-off that I suspect makes DRK better on paper, but in action healers would prefer the other classes due to the more stable damage income.