Yeah, I wouldn't be surprised if the biggest reason was their helmet, but imo they also suffer in killing except in the case of throwing (but as I've said, this is all from my personal observations rather than any actual data) so I think that their proficiencies should be increased as well.
Yes, the player can probably make them a great unit, but I think that a unit requiring micromanagement to be as competitive makes that unit worse (since the management also takes away from player's personal combat effectiveness).