Lux as a whole is not overpowered, but she falls in a small category where she is very strong in low elo and the higher you get the worse she gets. The complaint that she is not good because distance closing champs can get her easily make no sense because that is how she was designed. Her trade off for long range spells, longer cool downs and high burst is that she is one of the least mobile characters in the game.
P.S. http://champion.gg/statistics/#?sortBy=general.winPercent&order=descend&roleSort=&search=
This is a link to the stats of all characters compared to each other per role. I just wanted to make sure you know that Lux has almost a 49% win rate in mid and only a 46% win rate as support, which is understandable since she is designed as a hyper carry character so giving up farm and kills on a character where even the game suggests you build Mejai's is probably not the best. Also, Viktor's win rate is 46%, which is lower then Lux on top of being both close range and immobile, which by your stands is worse then Lux even though you say he is better.