The reason why TH9 war balance will be lost is that with the coming update TH9 3 stars should be on the verge of guaranteed every single time. This will be made possible with the additions of Dark spell donations, 30 extra seconds for raids and the inevitable TH11 level troops that will be able to be donated to TH9s, by TH9/10s with clan perks.
The attack that will be able to be a guaranteed 3 star is an air raid with two variations. Both include 4 lightning spells and 1 earthquake, along with a donated earthquake with the new dark spell donations. These are used to immediately destroy 2 air defenses right at the start of the raid. (2 lightning and 1 EQ per air defense kills it)
This leaves 2 air defenses, the queen and the CC to deal with. From here it is fairly simple. A small killsquad with 1 or 2 if needed golems, some wizards and heroes are used to kill the queen and cc and an air defense if possible and then the air raid begins.
The variations can be LaLoon or Drags or some combination of both, base dependent but vs 1 or even 2 ADs these attacks are virtually unstoppable.
Add in the 30 seconds of extra time and no one should ever run out of time. Add in the donation of TH11 level dragon/loon/hound/etc. that are bound to exist in the future and this type of attack will never fail.
TH9 will no longer be competitive for people who already can 3 star in the current state of the game. 3 star attacks will become the norm and be expected on every attack.
** *TLDR: TH9 will become akin to TH7 and 8 where 3s are much more common with one specific type of attack and no variation
TH9 balance is gone :(
(time to go to TH10 I guess)****
EDIT: Just to clarify, this post is meant for people who already can 3 star at TH9 successfully and for people who understand the concepts/logic behind attacking like this and executing this style of attack. Obviously more than just the "Casual" player. Any semi serious th9 war clan could execute this strategy with the Current state of the game and get 3 stars with an absurdly higher success rate than the current norm