There is no downtime to lulu shields with max cdr.

Meaning she can keep her adc with a permanent 400 extra hp. There is no opportunity when the shield is down so you have to brute force your way through the shield if you want to kill adc. Not to mention she can remove you from the game for 2.5 seconds and can give her adc 600 more hp with her ult. Reduce her shield duration to 3 seconds so their will be an opportunity to engage on adc.