Outcast NPCs do less damage to the shield, meaning they don't often drop it (a few of them have infernos though).
Corsair NPCs can drop the shields much quicker than any non-Gallic NPCs, so they feel like they're stronger.
It's not only the shield ... I wouldn't be supprised about that, with all the Tizonias and Coladas on them, but after my shield was gone, the Stork's hull with an armor upgrade dissapeared in less then 10 seconds, while being attacked by 4 Titan NPCs ... this is what bugs me ..