Spine Banks are definitely the way to go, just to add a few more shots. I will say that I have used thornback before on a CC oriented carnifex against Death Guard in a take and hold mission. Death Guard are fearless and their magic number is seven. With symbiote rippers I outnumbered him pretty badly. After losing combat he had to take even more saving throws. The squads just seemed to melt away.
Anyway, I would only use thornback on a dedicated melee carnifex, in games against fearless opponents.