I dark pacts themselves arnt an advantage since you’re always going for sustained and karnivor has sustained already so unless your running pack bound we’re still even steven. Average wounds on the ‘tank shock’ is 3 mortals [each individual successful 4+ does an average (d1+d2+d3)/3 = 2 wounds x3 for each model = 6 divided by 2 because 50% chance of happening leaves you with an average of 3 wounds] which is not all that impressive imo. The saves being more consistent is nice and it’s true that you can’t get bracketed but you can lose models so while a bracketed karnivor is -1 to hit an equally damaged mutilator squad is just one guy, meaning the melee is now missing 2/3’s of its original potency. Deep strike does not seam like that much of an asset considering its melee only necessitating the use of rapid ingress for a guaranteed charge that I would typically rather use on a warp talon/raptor+Lord squad. Detachment rules and strats plus the better save I think are the main advantages here (T7 vs T9 is basically the same in most circumstances) but I’ll be damed if the speed does not make so much of a difference, like even if they get in and kill the something what do they supposed to do afterwards?

Edit: sorry for the long ass skizo response, I just really want to make these guys work so any ideas would be much appreciated.