As described on this reddit post, one of the comments:

No single mechanic in turn 9 is significantly more difficult to learn than the ones in other end-game encounters, but the fact that ther are four fucking phases, each with wildly diferent movement/positional reqirements means that the only way to make progress is to play with people who have experience up to the same part of the fight as you do. Which is unlikely to happen when you're grouped with 7 random people. SE has done a good job on a lot of things in this game, and it's a well-put-together encounter, but how could they think that DF groups would be anything other than frustrating lessons in futility 99% of the time? I can only hope that SE learned from this when they were designing Heavensward's end-game raid.


The fact that making a mistake on any one simple mechanic means you have to start all over (and probably get kicked out), and that you can't even practice the later phases unless you and seven others can perfectly execute the ones before it is absurd considering the gear rewards (that you only get two of) at the end are now obsolete.