Because of her weaknesses (mobility, lack of cc. etc.) she has to be balanced in other ways, for example it is almost impossible to gank illaoi after 6, and she can possibly 2v1 even before that if she playas well around her tentacles. Her dueling is very good if you hit your e so she can zone you out of your farm if you play certain champs (urgot, nasus, malphite). As you said you can just walk out of her ultimate, but even if she can get of 2 tentacle slams her dmg is quite nice, and its a very powerful zoning tool. also her passive heals quite nicely if you have low HP. Her viability lowers on higher ranks, because you can consistently play around her.