The fact that you can successfully tank an easy story quest dungeon with no problem isnt the issue, considering even a melee can tank these dungeons and because of that a healer might say "you werent squishy" to the melee.

The issue comes from comparing it to other classes, you look at other tank classes and then you look at previous high level content (aka coil) and yourself how would DRK fare in this content? Thats when issues arise, considering other tank classes had some issues or needed a high skill level to be viable, currently DRK seems to have a lesser kit all around which means you have to work even harder to the point where you ask yourself is this worth it? And why not tank with a stronger class?

I dont mind the challenge but the keyword here is balance