I think the theory of the damage behind this item is a lot stronger than people think because it serves a niche that ADC's don't fully appreciate.
There are two important things to note about Runaan's that make it worth considering over other items: it give more attack speed than any other item in the game, and it has a passive that essentially allows you to attack two targets at the same time, each dealing 50% of the damage that you would normally deal, and applying on-ht effects.
The passive is, in particular, a good point to look at. Each of the bolts deals 50% of the damage you normally deal on per bolt, meaning that you essentially deal 100% of your auto attack damage every time you auto attack someone with at least two allies near them, and when you count your auto attack, you essentially deal 200% of your auto attack damage every time you auto attack. Since 200% of your auto attack damage is how much damage you deal on crits(without IE, at least), that means that, with Runaan's Hurricane, you essentially have 100% crit chance, but your crit damage is split between three targets. If you take on-hit builds, that number skyrockets to 300% damage on each auto attack because your biggest damage source is your on-hit effects, not your physical damage.
What does this mean in practice?
If you build Runaan's Hurricane, then you need to build around it, not shove it onto your normal ADC build. Traditional ADCs only build one attack speed item and Berserker's Greaves. If you buy a Phantom Dancer, then you have a total of 75% bonus attack speed, while Statikk Shiv gives 65%, while on more bursty builds with Blade of the Ruined King and Youmu's Ghostblade, you would have 40% attack speed passively (65% with boots) and 80% when Youmu's is activated (105% with boots). Runaan's would have no place on the burst build, but on the traditional build it could work. Let's take a look.
If you replace it on a normal ADC build, you may be better off trading out your berserker's greaves for either a more defensive boot pick, or maybe you want to pick up Boots of Swiftness to account for the movement speed you lose between trading out the Zeal-based items for a Hurricane. This would leave you within a range of attack speed that you would normally have on your traditional ADC build.
But what is an ADC without damage? With less crit chance due to taking Runaan's over Zeals, Infinity Edge becomes a less attractive buy because you don't have the crit chance to consistently apply the crit damage passive, so that inherently leaves it out of a Runaan's build. That us with a few options for damage items, but they can be considered pretty notable. The Bloodthirster, Essense Reaver, Last Whisper, and Black Cleaver can all potentially take a slot for your damage item, Blade of the Ruined King and Frozen Mallet can push you onto more to the on-hit effects of these items, while Mercurial Scimitar and Hexdrinker/Maw provide a more defensive approach to your item build. With these factors, the ideal build would most likely be: Bloodthirster, Runaan's, Scimitar, Whisper/Cleaver, your choice of boots, and your choice of defensive item (Banshee's, GA, etc.). This build inherently trades damage in duels for AoE damage in teamfights and a little bit of extra tankiness due to applying lifesteal on multiple targets and the ability to run tank boots in your build, but is it worth it? It's just a theory.
The other notable build to try Runaan's on is to incorporate it as an attack speed item onto a modifies blue build, with Muramana, Essense Reaver, and Runaan's Hurricane being the three core items. The theory behind the blue build is that you have the mana refund on Essense Reaver to apply infinite Muramana procs, the the idea behind Runaan's is the apply the on-hit effect to the entire enemy team. Combine this with a third damage item (which can now include something like IGB with the Muramana passive), your choice of boots, and a defensive item, and you should have a pretty solid build. Does that mean it's good? It's just a theory.
70 % Attackspeed
UNIQUE PASSIVE: Your basic attacks fire minor bolts at 2 nearby targets, each dealing 10 (+ 50% AD) physical damage and applying on-hit effects.
(and upgrades).
. Sadly this combination also is far from practical since it lacks flat AD as well as crit chance which means that it's horribly inefficient when used against squishy champions.
(E)
(Ultimate) , or possibly 
(Headshot) or 
(rockets).
Sadly theory and actual gameplay differ a lot, resulting in a situation where Kayle is the only one of these four who really can make practical use of it since she does significantly benefit from attackspeed, yet not at all from crit or attackspeed which carries urgently need.
(if really trying for the optimal Runaan's build, go 
instead. If for some reason the enemy has no armor, take 
)
had a viable upgrade it might work wounders with 
. You forgot 
with a 
and on rarer occasion a few others for even more niche scenarios. It needs to be remembered that Teemo is one of the most flexible champions, able to adapt to any scenario building AD, AP, Bruiser, even tank as necessary.
or
. Basically it is only good with adc's who love on hit and AS. The problem is that only somebody like 
, or maybe 
not