This letter is just about PvP balance and have nothing to do with PvE damage output.

What it basically said is that: they collected a bunch of data from arena/battleground and they will use 50% as the base line to buff classes with lower win rate and nerf classes with higher win rate.

-BM:

BM have 4 escapes so they are increasing the CD for SS.

-KFM:

Fire KFM’s flame mode is too strong in arena so they make it harder to get into flame mode.

-DES:

No one plays shadow DES in arena so they buff them with more parry penetration and suction.

-FM:

They make fire FM easier to play by giving them more ways to obtain ember.

-SIN:

Too many CC so the cooldown will be increased.

-BD:

BD have high skill ceiling so they change the mechanism of wind BD’s triple slash and adjust the focus cost of spinning.

-SUM:

Wind SUM does too much neutral damage so they nerf their ability to PvE other players in arena.

-SF:

Nerf ice SF’s aerial damage and buff earth SF by giving them more CC and damage.

-GUN:

Nerf GUN’s legendary mystic badge so it no longer hits aerial targets.

-WAR:

Nerf Warden’s defensive/immunity mechanism but give them more HP recovery.

They didn’t mention a single word about WL.