Trying to learn the champ but dont understand how a midlaner should choose her instead of other assassins.

Emerald 3 last split as jng and support, I usually play mid and liked the concept of the champ so tried to learn it. After tons of games, i cant understand how she could be a good champ nowadays. She loses to other assassins since she does less dmg, has no shield or heal to win simple trades; cant compete with mages since a simple cc nullify the whole trade and take the whole combo immediately after. Why should I play her instead of diana since they are pretty similar but diana has shield, aoe cc, spammable dash and low cd ranged q? Really want to understand what I am not getting of this champ.