Keep in mind this discussion is about dungeons, sure they're meant to be casual but this isn't a warrior in "raid" discussion.

Warrior in Dungeons is unbalance-able, You can make tanks pull more, make mobs hit harder and warrior would still fundamentally busted in AOE situations, It's current design with it's AOE healing is so ridiculous and it's every 25 seconds lasting 8 seconds, with the shortest invul that works well with BW, more sustain and defensives as backup that you'll rarely need to use anyway.

Warrior in raids is balance-able but likely a little overtuned for what it brings, also it's design just doesn't really make much sense being the "tank healer" even more then paladin is weird for the "berserker tank" Personally I'd want warrior to be a high damage, high hp and more selfish tank then current. I don't think warrior in raids is that unbalanced, but it's still kind of really good considering its low skill floor.

Is sustain a issue? No It's not a Issue, the issue is fights not allowing for high damage or high autos so that tanks actually have to make good use of that sustain in the first place, Again warriors sustain can be balanced, I personally would like a little bit of that sustain to go but I'm more concerned about it's "party healing aspects" And I'm not defending Paladin here either, I think we should look at it's magic healing and get rid of it for a extra way to sustain self or teammate but it's way less then the current magic healing you get from attacks.

Is dark Knight "perfectly fine in dungeons?" yes with current dungeon design it is, would I want dark knight to get more survivability/sustain? yes so we can talk about making mobs hit harder or allowing tanks to pull more, I personally would love slow over time sustain on dark knight as well. (maybe even give oblation a small sustain aspect as that ability is kinda just really boring right now lol).