Flood are morons in game. They juat stand and shoot,.they do not take cover when being fired upon, they run in a straight line towards you, sometimes they jump towards you so you can fill them with bullets as they fly through the air.
The entire strategem is to use sheer numbers to run at you overwhelm you, hence the name Flood. Brutes in Halo 3 were smarter than flood, and they were idiots. As far as a gravemind being smart and vicious, none of that gets reflected in actual gameplay.