In big numbers they can eventually kill us, if we are alone, but they still felt very lacking to me. If I had to give an example of what I expected a Thrall to move like then that would be the Stalker from Dead Space....hides whenever it needs to and suddenly it's ramning us to the ground.
But maybe I'm the only one around here with this opinion.
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They are actually pretty smart as is, I threw a grenade at their spawn room during the moon level and the ones that weren't hit by it waited for the grenade to time out before continuing to charge at me. Other games would have just had the Thralls run through the grenade and die (Which was my idea when I threw the damn thing)