There is a huge balance issue between quests and high level dungeons.
It's a completely different approach than DDO. Quests are all easy by design and scaled for solo difficulty even when not soloing. High level dungeons are immensely more difficult with high failure rates. I wouldn't say they are impossible to complete. They have much higher failure rates compared to EE quests or high level raids in DDO. I don't know if that is by design, flawed design or they are still trying to figure it out. It's just like DDO I am sure, once you learn the right positioning and tricks it gets easier. I haven't learned those yet, but I also haven't made it a priority.