Is Lux too strong? If so how?

Hello, I've been arguing with a friend recently that Lux isn't broken, she is just strong and A tier about in the meta. He disagrees heavily, saying that her ultimate CD is too low for how much it does but doesn't factor in that she NEEDS her ult to kill someone unless she is insanely fed or if you let her use her basic combo at least twice while procing her passive with autos give or take 2-3 times. I personally think she's good and balanced because she's a burst mage, burst mages are supposed to one shot squishes. Also she does horrible into tanks or anyone with enough mobility to jump on her and kill her before she can land an ability which brings it to the fact that if she doesn't hit her spells she is useless and on top of all of this she is very easy to gank unless she has a handful of kills under her belt and considering who the enemy jg is and the enemy mid laner. Thank you~ <3