Pyros
<Silver Donator>
It costs a lot of potency if it doesn't pop. Granted it pops on any relevant content for the most part, just not on like, dungeon bosses and shit. On dungeon trash I think it's a bit worse than some of the other cooldowns too if you got a large enough pull, because 15-20% of all incoming damage for ~6sec tends to be better than 25% of your health with enough shit hitting you, although TBN does scale much better in combination with another cd(the 25% health becomes extended if you have rampart up more or less, while rampart+heart/intuition stack multiplicatively make them worse). But dungeon trash is just that, trash, so it kinda doesn't matter all that much.TBN doesn't cost any resources either... there's never a reason to go below 3000 MP on DRK so you always have enough to use TBN.
I'd say the biggest concern is how late DRK get TBN compared to other tanks side cooldowns. WAR get Thrill at 30 and Raw Intuition at 56, PLD get Sheltron at 35 now although the trait to make it much better only comes at 74 but they still have passive blocks too at all times and GNB while they get Heart only at 68 so almost the same as TBN, they get Camouflage at low lvl as an additional cooldown which actually works on everything, unlike Dark Mind. It's not a concern for max lvl play, but lvling a DRK you're very lacking in defensive options before 70, since you basically only have Rampart/Shadow Wall until then, and Dark Mind sometimes.