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.
On the moon mission on hard, when that door opens, it's like the flood gates to some one with a scout rifle. I was running and using my jetpack for a solid 6 minutes.