Sera T3 mobile shield isn't that much different to a fixed UEF T2 shield, despite being mobile and able to go on water:
T2 UEF structure: 600 mass, 9250 health, 120 regen, -200 energy
Sera T3 mobile shield: 720 mass, 10400 health, 133 regen, -175 energy
So costs 20% more, for 12% more health and 11% more regen and marginally less energy consumption. The size isn't much smaller when you factor in the mobile shield is a lot smaller than the fixed shield (i.e. you can fit units in the area that a fixed shield would take up). However, it's mobile, making it far more versatile.
Other factions' mobile shields are harder to use since they have a much smaller shield size (needs more micromanagement), and the shield is a lot weaker (so you get less benefit from using them, and bombs and arti shells will quickly break through them).
Currently if trying to kill a sniper bot under a Sera mobile shield with strats you'd need 4 strat bombs. If the mobile shield health was reduced to 8250 from 10k (17.5% reduction) then most factions could kill with only 3 strat bombs. The mass cost could be reduced slightly to compensate (e.g. a 12.5% reduction to 630).
The shield size could also be increased - while this would be a buff in some scenarios, it'd also make the mobile shields more vulnerable to T3 mobile arti fire, and mean multiple shields would run into the shield overlap and aoe damage issues.
I.e. if they were to be nerfed, it'd be nice for sniper bots to retain a key role as a counter to slow moving ground units such as percies, while in turn having clear weaknesses to units such as mobile arti and bombers that can only be lessened but not removed through the use of mobile shields.
If the health of the Sera sniper bots was to be increased, it could also make T3 mobile arti much weaker against them when they were without shields, since UEF deals 750 a shot and Sera 700 a shot (i.e. they can 1-shot an unprotected sniper bot at the moment, since the sera health is 700)
Another option is to make them more of a glass cannon - increase their initial damage, but increase their cost as well