[Dragonspine]
Coils and sepulcher are great dungeons (I have not seen the new boss yet) but no one farms them due to the gear being worse than khati epic. These dungeons are the hardest ones in the game, yet drop mediocre gear while farming flame/vortex 100 times gets you better gear. The only worth while piece is the talisman that has around 250 protection. If they dropped gear better than khati epic, (maybe even a bit better than t4 because you need a t4 tank for last boss in sepulcher) far more people would farm these dungeons.
[PVE]
Generally I think more competitive gear (T4 quality or better) can drop from the more difficult dungeons. There are lots of people who prefer not to play with 23 people, especially when one person could wipe us all.