I never realized engineer bonuses stacked. Hot damn.
Yeah a lot of the bonuses stack. My last run with Chaos I had 3 Exalted Heroes with Infernal Dominance - +15 melee attack to everything in the army total, then of course add on the chosen buff skills from the Archaeon trees. Chaos Chosen 2 hand weapons with over 100 melee attack SLAUGHTER everything in the game in short order.