The issue is that her passive + Q combo is a stat check monster. The only champion that has even any slight hope of surviving her combo post 6 is the top laner, and even then if Akali has ignite they’ll probably die too.
Now, what pushes her out of Fiora and Camille’s «oppressively strong but not uncounterable» domain and into duo mid Yi’s «game breaking» domain is said stat check burst combined with all the following facts:
Skill set naturally synergizes with sustain items that provide both insane burst damage, stat scaling and durability
No mana costs, extremely low cooldowns = Pantheon style laning play
Point & click spells means you cant win a duel through positioning
Hybrid damage, hybrid scaling = extremely difficult, inefficient and expensive to build items against
Extremely long range dash that deals bonus damage, places her on the other side of the victim, applies spell effects like healing and holds multiple charges
And ultimately what makes her one of the most unfair champions in the game to play against
Invisibility zone with extreme duration, low cooldown, a short range blink, an absurd movement speed boost that also slows opponents.
In essence, the champion:
Has very high item efficiency, with high build flexibility - gains much more strength from items it builds than most other champions in the game. Can also build and effectively utilize nearly all the items in the game. Zhonya’s hourglass practically eliminates window of opportunity between stealth shrouds.
Has low outplay potential / skill requirement - while i’m not claiming she has no learning curve or skill potential, champion is extremely, extremely much harder to play against than it is to play. Where the akali player can easily enter the double digit kill score with very little practice, thinking, decision making or timing, her opponent needs to be quite a lot better at the game overall to even stand a chance at any counterplay. A gold player can typically make platinum and even low diamond players struggle through stat checks alone.
Snowball power - like every stat check champion and hybrid champion; almost impossible to shut down if ahead, because she can build new items faster than her opponents, which links together with item efficiency detailed above to propel her even further ahead. Opponents already have to build hybrid resistances - while Akali stacks damage, opponents need to somehow find gold for THREE resistance types* BESIDES their core build.
Indistinxt class, jack of all trades - where she is certainly an assassin, she operates more like a sustained duelist, skirmisher AND burst mage all at the same time - and with a defensive item or two, a drain tank and a diver.
Wannabe juggernaut - Like Illaoi, Nasus and Cho’gath her counterplay often boils down to «outnumber her» which means any true team fight is inherently lopsided in Akali’s favour. You need multiple people to take her down even if the player is not particularly good, which opens her team up to having other powerhouses that require numbers over skill to take down. She is much more durable than any other assassin in the game, being the only decent challenger to tank ekko back in season 5 and tank Katarina in season 6.
Panic button that solves everything - As she does not have freeform mobility like Ekko or non-takedown resets like Katarina she’s not quite as unbalanced as they once were - but in return she has one of the most toxic gameplay patterns in the game thanks to her shroud, which lets her repeatedly pop in and out of untargetability whenever she needs to. Using this spell is never, ever a bad idea - there’s no decision making, no tradeoff. Want to cross a wall? W. Losing a trade? W. Want to poke/farm in lane? W. Getting ganked? W. Spells on cooldown? W. Want to run away? W.
Her shroud is an ability there are only TWO ways to reliably counter. 1: pick twisted fate and use his extremely long cooldown ult to reveal her and 2: pick kleptomancy and hope that RNG lands you an oracle’s elixir. Both of these are not just playstyle defining methods but can potentially be game defining. Akali’s primary counterplay revolves around her opponents being FORCED to change and tailor their entire play style, build and game plan JUST to her presence alone. Riot has deliberately moved away from these things in the past, but not with Akali as they simply couldnt see any way to fix these problems without a full gameplay rework.
Tl;dr akali has an extremely unhealthy playstyle that is something like crossing Fizz, Jax and Katarina.
*health, armor and magic resistance
require to hit her Q in order to reset her R, when 
has 3 charges on it and reset upon killing an enemy
