I'll get roasted for saying so, but... ah well, the problem isn't with Akali, it's with the balance team themselves. Akali and many others could theoretically be balanced, even with their unique kit mechanics like true invisibility... but because Riot makes all the money in the world (for not doing their job)...
Your issue is with sustain. This problem affects: Akali, Aatrox, Caitlyn, Cho'Gath, Darius, Draven, Fiora, Illaoi, Irelia, Jax, Yi, Vladimir the list goes on and on... but... if you can name me a broken champion, chances are they'll be balanced out by balancing sustain. Unfortunately, many people ignore that and choose to blame the champion themselves like you've just done and, so, the issue never gets resolved.
You notice Fiora? Darius? Yasuo? Akali? Aatrox? Lots of healing, right? It's almost like the healing is stacked onto their kit, sometimes in ways that don't even make sense (Irelia) and the champions who don't have self-healing are in turn overloaded as fuck because they, otherwise, can't keep up. We have CertainlyT to thank for this, but fortunately he's not going to be designing LoL anymore after releasing yet another overloaded champion balance nightmare.
Edit: in some cases, the healing IS literally stacked onto their kit (

or Fleet Footwork/Conqueror) and a 40% healing reduction just isn't enough to deal with something like that.