@maudlin27 I think you're underrating the shield by rating it purely on its stats/mass for a few reasons:
A big shield HP pool is super valuable to the UEF roster due to their reliance on Percies which are very vulnerable to AoE from Ythotha, Megalith, Fatboy, and SACUs.
Percies are also vulnerable to overcharge which is imo a bit too early to be building shield SACU vs ACU, but Sera SACU can have OC that the shield would be good against. The shield generator also does not die to a single OC like other mobile shields.
Due to overspill shields have a cap of 3.3x their HP in terms of how much damage a shield ball can block, so having 2.2x the HP of Sera which is 3.3x UEF and 2.85x Aeon lets even just the first shield have much more effective HP than stacked mobile shields. Sure you can micro shields in/out or off/on to mitigate overspill, but vs burst damage there's no time for micro, which plays especially nicely into UEF's roster countering the air snipes that Fatboys experience.
The single large bubble takes less damage from AoE than stacked small bubbles. For example billy nuke definitely would not go straight through and kill the shield generator, while it can 1-shot giant stacks of T2 shields and 2-shots stacks of T3 shields due to how AoE interacts with multiple small shields defending each other to deal extreme damage.
The SACU also has buildpower, can be upgraded with sensors, and has tons of HP letting it survive to recharge instead of dying like normal mobile shields.
