Ok if you want proof I will give you the computations next time. But his damage amplification (15%) is worthless. Assuming you deal 1k damage (which is hard to achieve really), you only get 150 additional damage, which is way less compared to other assassins that have damaging abilities based on the enemy's % hp. Heck, even swain got more damage amp at 20%.
Compare that to khazix's evolved q damage, poppy's q, fizz's w and zed's passive. Which deals about 200 damage for example as 10% of enemy's 2k max hp. I just think this is more effective in assassinating as there is a guarantee that you will take away a chunk of health from your target.
if you disagree with me, you are free to comment. lol
edit: ok i realized he does do a good burst. i take back what i said above lol.