There are 4 shield mods, and 1 armor mod.
Shield booster, energy to shield converter, shield reinforcer, shield ionizer. There are 4 shield booster items, two of these give +100% shields or more, and one reduces damage by up to 30% for a given damage type. All of these provide either massive buffs(booster and converter), minor buffs(ionizer), or game changing functionality for large debuff(shield reinforcer).
The armor mod provides a minor buff to hull 30%, and a major penalty -- 300% damage.
This system mod is seriously bad even with the armor buffs that have come in the new patch(I love being able to make a non-shield build, its great!). In a firefight, I will only know if i'm taking the wrong kind of damage while the damage is happening, and the system must be permanently installed to work. There's no way to scan an enemy for knowledge of weapon type, which means I cannot choose to run away. The only option if I realized I was dying is to destroy the system and unequip it.
I dont believe the buff to armor was needed if armor had similarly costed system upgrades. But considering the shield ionizer is roughly similar in functionality(30% damage reduction for no penalty), a 30% damage reduction for 300% penalty seems very harsh. Make it 30-50% damage penalty instead, and this could be an effective tradeoff.