Parnell requires sacrificing HP, getting in close and aiming for weakspots or headshots with a shotgun that has considerable recoil. He puts himself in danger of the monster damaging him, swiping him away or simply jumping away and negating his burst. Finally, unlike all the other assaults he brings no added utility for the team in the form of mines or grenades.
Torvald does absolutely huge burst without having to get in close, sacrificing ANYTHING, while being able to spam it without reload while being able to stay safely on a ledge away. He also brings utility for the team through his grenade.
Am I the only one who sees something wrong here?