So I played a couple matches earlier tonight on the PBE as Teemo, who already has a powerful on-hit in his E. Runaan's Hurricane is a great item, and synergizes a bit too well with Teemo and I was curious why on-hits carried over with the item.
As an example, I built Teemo with Sorc Shoes, Malady, Rabadon's Deathcap, and Runaan's Hurricane. Quick in-head math is telling me that I'm sitting around 200 AP after masteries (I had no AP from runes, only spell pen), which is pretty nice. With Teemo's E (Maxed first btw) having a .4 ratio and Malady now having a .1 ratio on all auto-attacks, I'm looking at over 100 magic damage getting added to every auto-attack without the base damages from my E or Malady, 170 when you factor those in. Also, I will have at least 1.7 AS (Level 9 with 120% AS added, gets higher as I continue to level and I'm not likely to have these items before level 13 even). Also my natural attack damage is roughly 85-100 around that point.
The damage output for this pretty simple AP/AS build would be strong by itself if it was all focused on a single target (255-300 on the primary target, with a 1.7 AS); instead I'm doing that much to the primary target and then only about 40 damage less to the secondary opponents, since the build is doing damage almost entirely via the on-hit effects. This seems a little outrageous since it essentially obsoletes every other Teemo build through the use of this single item; I would have higher burst output with a full AP build of Teemo, but single-target only in a fight and a low AS, while an AD Teemo with this item wouldn't have nearly the damage output on the secondary targets.
Why does Runaan's Hurricane apply on-hit effects, especially stuff like Teemo's poison? It is a great item on Teemo (An /AMAZING/ item), but I am worried that it's so good that it is choking the build option of pretty much any champion who can buy it. I faced a Twitch earlier tonight that used Runaan's in conjunction with a Blade of the Ruined King, and watched as he melted our entire team in fight after fight while life-stealing thanks to the bonus effect of the Blade.