well, her strengths are in her bursts and ability to chase, but in reality, she is easy to focus, becomes visible for a long enough period of time when she attacks that you can even auto attack her. She is only invisible in her shroud and she deals entirely magic damage
katarina does more than akali but she's squishier due to not having spell vamp and suffers from th e same magic damage problem
Akali can do wonders in a team fight because she can hide and fight at will, but if the fight moves off of her shroud, she loses her durability, movespeed bonus, and invisibility
if you max cooldown reduction on akali, her E will have about 1.2 seconds on it and allow for 5 uses before her energy is used up
personally, a good tank or fighter or mage wrecks akali, examples are chogath, ryze, sejuani, annie, riven, brand, xerath, ziggs(his minefield is a big deterrent), mage kogmaw, then of course alistar, who can heal his team of some of her damage and can stun in an area of effect while being tanky enough to engage the whole team for a bit with his ult...and i'm tired of thinking of others XD , oh, malzahar is a big one
malzahar can make his W cause 15% of anyone's health per second in an area of effect and his ult can do over 1000 damage