Every soul collected is one more bonus magic damage on your auto, not to mention the ad scaling over time. If it gets to late game and you end up with 200+ souls, just rebuild some atkspeed and take heads?
if you are active on the map you'll average about 100-150 souls at the most
and he doesn't keep his 1:1 returns, it drops off to .5. He does become fairly tanky, by having good health gains and his free armor isn't to be scoffed at, but in terms of damage he doesn't scale as well as many other champs.
You can take advantage of his Magic damage insanity with stacking ad, but it is flawed outside midgame. He has short range, he needs to refrain from attacking to get the massive strikes, although his per hit isn't horrible with a decent amount of ad, and if you try to take advantage of his magic damage you won't be building to take advantage of crits like other adc's do.
He is a very good lane bully with some ad, but he suffers from similar issues as something like adc teemo, albeit his on hit magic damage scales with ad. Lategame he could potentially oneshot a squishy, but he has no way to do that without dying in the attempt.