Wuju Style (E):
Active's Flat damage increased to 12/19/26/33/40 from 10/15/20/25/30.
Active's AD ratio changed to 25% bonus AD from 10/12.5/15/17.5/20% total AD.
This should be a total nerf to the amount of damage he'll do when building tanky. That's why Riot is doing this.
(Taking info from the League wiki about Yi and common offensive items and only taking relevant stats)
Master Yi starts the game with 60 Base AD and it scales to 111 Base AD at level 18.
Master Yi sometimes builds Guinsoo's, which fully stacked grants a total of 54 AD and 72 AP plus 20 (+15% Bonus AD) + 7.5% AP as on-hit damage.
Sometimes he buys Blade of the Ruined King, which grants 25 AD plus it's 6% current health damage on hit.
Right now, in patch 6.5, Master Yi's E is dealing:
- 30 + (22) true damage on hit at level 18 w/o offensive items
- 30 + (33) true damage on hit at level 18 and a fully stacked Rageblade, as well as 28 on hit Physical Damage and 5 Magical Damage on hit
- 30 + (27) true damage on hit at level 18 and a blade of the ruined king, plus 6% current health physical damage on hit
- 30 + (38) true damage on hit at level 18 and both the two offensive items plus their on hit damages
In 6.6, Master Yi's E will be dealing:
- 40 + (0) true damage on hit at level 18 w/o offensive items
- 40 + (14) true damage on hit at level 18 with a fully stacked Rageblade
- 40 + (6) true damage on hit at level 18 with a blade of the ruined king
- 40 + (20) true damage on hit at level 18 with both offensive items
As you can see, he will not being doing "wayyyyyyyyy more damage", he will only surpass his previous damage numbers if he has over 100 Bonus AD, which takes at least 2-3 normal offensive items that a Master Yi would build. This effectively makes him need to build Full AD to get the same damage numbers or more than he has right now building tank/bruiser, in patch 6.5, which is the primary reason of this change on the PBE.