"I hate this fight somewhere deep in my soul."
-- random DPS on my first Thornmarch EX clear

The reason people hate this Trial is because there are checks for every role that when missed = instant wipe and they happen repeatedly and at a fairly high frequency throughout the entirety of the fight.
Tank doesn't position King correctly = dead.
Healers don't remove debuffs = dead.
DPS don't stop Flare or Holy (or heavens forbid kill a moogle early) = dead.
Then there's the fairly strict DPS check at the end (getting him down to 0.01% and wiping = priceless)

Then on a more personal side, that fight is utter HELL as a SMN..., trying to control your DoTs + your pet ;_;