If by shield boosters you mean Guardians Shield Reinforcement Package, then they are useful indeed, but they do cost power and you cannot deactivate or set power priorities on them
So you have to make a conscious decision:
- more hull (with little to no penalties) or
- more shields with some penalties (more power which also mean more heat, and the prospect of having your powerplant shot leaving you stranded because you exceed 40% power)
For example i do not use them on my vulture, but i rely heavily on them on my alliance ships
There are also shield boosters that go into utility slots - they are sort of useful non-engineered and really marvelous when engineered.
And there are also the Shield Cell Banks that go into internal optional slots and act like a battery to recharge shields.
They do cost a lot of power, they really heat up your ship and they have limited charges. Also the ones below size 6 are not really impressive in terms of recharge capacity