Here is why Yi is much stronger now than he was before.
Fervor of battle.
It essentially gives him a free 100% damage increase late game with 0 items. Fervor of battle adds as much damage on-hit as his entire base AD late game while also synergizing with double strike and devourer.
This means he can afford to take tank items without affecting his effective damage.
Before, Yi could either be tanky and do good sustained damage, or be glass and do huge sustained damage. Now he can either be tanky and do huge sustained damage, or be glass and 1 shot people.
This is why I think the new masteries are unbalancable. As we've seen, there's always been an abuse case: Graves with thunderlords, Trynd with Warlord's, Yi with fervor, now Udyr with Strength of Ages rushing Zz'Rot.
Broken
Broken
:)))))