I'm sure it works because of his ridiculous 4 second stun, 360 physical damage nuke that increases with armor reduction, but he has better potential late game than a mid-game ganker.
If your team is full of carries, he'd probably fit well as a ganker, but if you're even in mid tier games, he'll be left to be the carry.
By the time you hit 30 minutes, your mid-game ganker alchemist potential falls off heavily because you could have had an AC and BKB by then. Hit 35 minutes and you have a basher to somewhat counter the other team's BKBs. Hit 40 minutes and you'll have an AC, Abyssal, BKB, Treads, and a 4K gold item + buy back.
Compare that to a ganker Alch who has an MoC, Blink, Necrobook, and boots of speed.