Shield duration nerfs nerf ardent on Janna/Lulu/Karma, piling on nerfs beyond those to ardent itself
Shields are the ardent delivery system for Lulu and Janna and Karma. Longer shield durations enable safer delivery of ardent, because it's less likely the shield will be wasted just to apply the buff. However, with lower shield durations, ardent becomes a lot worse; using your shield to buff your AD is a lot more likely to result in the shield itself getting no value.
Given how much of the power of Janna/Karma/Lulu is in Ardent, this strikes me as potentially hitting them harder than intended (proportional to the duration removed -- so Lulu has it the worst). Also a bit of an indirect ADC nerf, too.