Ability haste is much too abundant in the augment pool and leads to a less engaging stand offish gameplay. Between infinite recursion, eureka, archmage, shrink engine, recursion, bread and butter, bread and cheese, bread and jam, the 4 ult haste augments, the 3 pursuit of haste augments, purist caster, and probably even more that I forgot every single champ is able to get a haste augment every game.
This leads to the massive CC loops we all know and hate and constantly shrinks the windows people have to capitalize and play the game.
Some champions have massive area control abilities that paired with presence of mind's free near infinite mana just arent fair or fun for anyone. (Taliyah rocks, Hwei's entire lane wide fire that lingers till its back up).
The only thing you can do is to dive into a setup where ap shaco and teemo set up 20 traps with their 300 haste. You somehow have to weave through 1000+ range abilities that literally stun and turn your character around on a 4 second cooldown. When you inevitably cant cross the 3 screen distance without landing a max range snowball they had 5 seconds to react to just remember the enemy is more skillful.
Its mindblowing that as a tank against 5 mage teams you're not safe, because theres no lack of dps when everybody has 300 haste.