But there's a cost for that - it's extremely unreliable, has no bonus effects, and shields for much less unless you get the perfect conditions.
It's not a point and click like Janna shield, Orianna shield, Morgana shield, etc. It doesn't have an additional buff like AD on Janna shield, bonus armor/MR like Orianna shield, and CC immunity like Morgana shield. It only shields for half as much as those other shields the first time around, and the shield amount doesn't stack, it only refreshes. To get the full benefit you have to be taking damage so that first shield pops, and then you have to be in the path of the shield when it come back, which is basically forecasting your position if you try to get the rest of the shield.
As for her E, the numbers on it are just bad all around. It has the lowest damages while having the highest mana cost of any similar waveclear on a mage midlaner. 10s cooldown at all ranks, at rank 5 has 240 base damage and 60% AP scaling.
Compare to, say, Orianna W with a 9s cooldown, 250 base damage and 70% AP scaling (plus moving her ball into position to cast it does damage as well).
Syndra Q, with a 4s cooldown, at rank 5 has 264.5 base and 69.5% AP scaling.
Xerath Q has a 5s cooldown, 240 base, and 75% AP scaling.
Ziggs Q has a 4s cooldown, 255 base, and 65% AP scaling.
Obviously these are not 1-to-1 comparisons, but even so it's not hard to argue that Lux's E falls far short of the other abilities. 2x to 2.5x the cooldown for 3 of the 4 examples here, equal or worse base damage in every case, and worse AP scaling in every case.
Orianna W is the most similar ability here, with comparable (actually better) utility - because it still has a smaller cooldown, a stronger aoe slow, an aoe speedup for your team, and continues to grants vision even after it has been "detonated." With Lux's E if you want the enemy not to walk out of range right away you have to detonate it immediately, which is how it's used most of the time. So most of the time, the utility in it is completely lost. Meanwhile Orianna's W leaves the slow field and the ball behind, so you get the (higher) damage AND the (higher) slow AND the vision. With Lux's E you get slow and vision OR damage.
Choosing Lux sometimes makes it feel like you're only playing half as much as other mage mids, because you spend twice as much time running in circles in the back waiting for your cds, and when they come up they don't even do anywhere near as much damage.