Basically, my Yi build is more fun and cowsep's is more boring. I just think crit is unnecessary, just go attack speed, you can get ur q up faster and you deal more dmg thanks to bork and rageblade passive. Also, wit's end gives you a lot of heal in teamfights. With lethal tempo, I've reached 3+ attack speed which scales really well against tanks. For instance, I 1v1'd a rammus late game, he had armour and etc, and cos of my attack speed, I absolutely shredded him.

Cowsep's build is good, but it just lacks sometimes, especially when you go crit, but the enemy has an item like randiuns, which just neglects a lot of ur crit dmg. I know ur thinking that items also reduce attack speed. But when you pair lethal and my items, you will still be above 2+ attacks speed, which is more than enough to win u the game.