To be honest, I do agree that Lux's kit is very strong and is borderline OP. I just don't think the ulti is the part that does it.
It is very well to talk about it being a skillshot - and it is - but people seem to discount that in a chaotic team fight, it is very hard to dodge it. It is also very hard to dodge it when it combos after other crowd control, including slows.
The thing is that the cc attached to her skillshots are very strong and while they don't do amazing amounts of damage, they can shut down melee characters very hard if used in a proper sequence. It also denies ranged characters the ability to kite and allows you to reposition much better. The fact that it's pretty much all decent AoE also helps it be very good.