It' s not OP, but it has an hyperscaling effect that makes her deal tons of damage if properly geared and allows hero to recover a lot of HP if she managed to deal damage, and coupled with the rest of her kit can cause not pleasant situations.
This means that is a passive skill closely tied to the amount of items you have, and if left to starve she can' t do very much in games ( even if in most of cases an hextech gunblade is enough).
This, coupled to her R, the stealth and the other skills allows her to faceroll the keyboard in 1v1 situation and kill an enemy even with low health ( as now a bit less, since the shroud does no more give free defensive stats) and to insta-shot most of targets, since most of the skills are damage-focused.
There is a really low skill cap with her since there are few creative ways to use her skill, shroud and ultimate ( most of time revolving around gaining time or making reckless assaults so spell vamp can proc) are the only one that requires a tactical decision over the fight.
You can make only few things against her: starve her in lane, oppressing her before she hits 6 ( and still a good akali player can gank othe rlanes afer that); asking the support to get an aegis so everyone has a base mr; having CC's that can be effectively used in a very few sec interval, when she dashes to an enemy ( not counting situation where she is picked off-guard, obviously) so she can be focused freely and save her victim.
Consider, I play Zyra support and many times i have dealt with Akalis in mid-late game while i roamed, killing them or making them lose time/being less effective, and no matter how much the player was skilled in every case i encountred, akali can' t allow skill expression too much and are all pretty much the same.