While the mobs might had before low health now it seems like you fight mini bosses not cause they have a high dmg output but more cause it takes forever to kill one.
But lets compare some numbers from the Magelo Clone...
Sebilis : lvl 55 froglok ilis acolyte = 15000 HP
lvl 70 Tolapumj = 350000 HP
Kael : lvl 70-73 Trash mobs = 70k - 100k HP
And now Umbral !!!
lvl 55 A Dark Assassin = 248750 HP
lvl 60 Rumblecrush = 42651600 HP DMG 7000 to 17000
Well i think something went really bad with the nerf in this zone ..