I have no clue what you're talking about. If you hit all 4 shots, that's 320+240%AD at level 6. The only other Ults in the game that hit that hard come from mages and full blood stack Darius and Cho'Gath. Now, take into account that it's also a passthrough skillshot that doesn't lose damage as it hits targets AND heals you as you're dealing damage, to the tune of 80+40%AD if you land all of them on one champ. At level 6, Heal heals for 175. You're packing one of the hardest hitting ult's in the game, with half a Heal minimum, which means you're kind of wrong saying that it is weak.
Since the damage is dealt in 4 segments, it might feel like it's weaker, since it's not moving their health bar down as fast. That's the only reason I have for why you guys feel this way.