I don't mean to devalue Devastating Wounds but I think for them to be really good it needs to be either on a weapon with an absolute ****load of hits (a bunch of outriders with a chaplain? The Redemptor build you're talking about? A full block of ten infernus marines?) Or on something that you'd be shooting at that target anyway, where the Devastating Wounds are a bonus rather than the requirement to be good (for example Railguns, where if you don't get that critical wound, you've still got a 50/50 shot they'll fail their invuln.)

An assault cannon just isn't either of those to me. Assuming Oath Of Moment is up, you're hitting with probably five or six shots - seven, perhaps, if you have Sustained Hits? - and have an expectation value of 0.8-1.2ish devastating wounds. Maybe two? But either those other shots are wounding on 4s or 5s and then allowing full saves... or who cares if you have devastating wounds up? Oh wow, my special rule killed half of one intercessor and the rest of the gun finished him off. wonderful.

I'm not sure I wouldn't take an Assault Cannon on terminators, mind, but that's mainly because I don't rate either the flamer or missile launcher tremendously highly either. On a dreadnought, as compared to a lascannon? Nah.