An explanation of why Yi is broken in the current meta.

First and foremost, I want to say that I don't think Yi himself as a champion is broken, but the meta has just made him that way.

_Back in the day if a Yi wanted to deal the type of damage he did now he had to actively work for it. Crit Yi builds wheren't uncommon and they took a while to achieve, giving his opponents time to counter build around him. If you didn't correctly counter build Yi, you melted in every fight and that's simply how it worked. This design philosophy was balanced, as Yi was a ticking time bomb to his opponents. _

However, the reason why Yi is currently abused int he current meta is that he doesn't necessarily need to build damage to do damage. Ever since the introduction of Conqueror every Yi i've been playing against has either built Guinsoo's entirely before finishing their first jungle item or just rush it right after. Here lies the fundamental problem with Master Yi, once he obtains Guinsoo's the amount of damage he dishes out is simply to much for any player in the game to handle. You might be saying "But you can still cc lock him in team fights" and I will go on to explain just why this doesn't work in the current meta also.

For some unfathomable reason Riot decided to turn Essence Reaver into an urf item and who arguably gets the most use out of it? Master Yi. Essence Reaver, alongside Yi's massive attack speed he gains from his ultimate, runes, Guinsoo's, and blood razor effectively make it so he can Alpha Strike every amount of cc you throw at him. Before it was okay, as he required kills in order to get resets, but when Yi is getting resets for simply auto attacking there lies the problem. It's hard to cc someone that continually goes untargetable legit every 2 seconds or less because of just how many auto attacks he naturally can get off in such a short time frame. This is where you separate the bad Yi's from the good Yi's, because they know how to properly use his Q primarily for dodging cc.

I want to finish this post off by saying that Yi's core problem is deeper than what I've previously stated and it's the fact that he doesn't need to build any specific stats to be effective, which is a horrible design philosphy. Once a Yi has built his Guinsoo's and Essence Reaver he can effectively out dps every champion in the game to an extent, making the need for more damage ill advised and causing more competent Yi players to build tankier items or ones that offer more utility (Death's Dancer, Titatnic, GA, Wit'z End, etc). Think about it like this, why would a Yi need to build armor penetration, CDR, attack damage, attack speed, or anything of the sort when his core items grants him everything he could want (which in reality are just 2)? When you have as much true damage as Yi, especially true damage that is so easily accessed, the need for anything else is pointless.

_You wanna know why Yi's damage is broken right now? Because every boosting strategy just involves a champion that can ensure he survives in a teamfight (Kayle, Lulu, Taric) in order to single handedly kill everyone. _

tl;dr: Yi's problem is the meta itself, not the champion.