Engie has anti armor and shield skills, perfect for highest difficulty.
So... what? (then again, my first (and only) play was Engie on the highest difficulty, and darn did these skills come in handy)
Warp kicks down Shields AND Barriers while dealing AoE damage. Drones are mostly worthless unless spammed (and then you have no tank or adept in your party). They also get some pretty bad weapons and don't exactly have the highest shields.
I'm sure you can go through the game on hardest with them, like any other class, it's just a much bigger challenge.