Tough question. She is 100% skill shot reliant, so it largely depends on how good both you and the enemy are. If you can predict your enemy movement - if you recognize when a Zed is going to Shadow-blink towards you or if you can just predict their movement patterns - then you will be much more reliable in landing your snare which is her primary means of safety.
Lux does trade early game safety for her late game, though. Like any glass mage, she needs her team to protect her, but the payoff is her insane late game scaling, range and AoE damage. A good frontline with a farmed up Lux can decimate teams, but if you don't have anyone peeling for you, then you'll be too busy trying to get away from the enemy to kill them. I would only recommend picking Lux if you have strong protection (Leona, Thresh) or really strong initiation (Malphite)
Lux, in my opinion, does do better when she engages rather than when she reacts. Try to land a Q rather than harass with E. Even without mobility, E is harder to harass with. While you will miss some Qs, if you land one, you can get a lot of damage off with Q>Passive>E>Passive
As for her shield, remember that it starts off giving her a shield, so you get the protection immediately. Then it refreshes when it returns. So the best way to use it is to cast it in the direction you are running. Cast it ahead of you, not behind you.