Duties are hard 90% of the time because someone or multiple people in the DPS role don't know how to perform their job to minimum dps expectations of the encounter. Then throw mechanics on top of that. Then consider once you've got all competent dps it still doesn't mean the Tank or Healer are on the ball.

This would be reduced if there were stronger tutorials, but the game kind of just expects players to intuitively know what to do, which clearly doesn't always work.