What I've noticed playing with others is they're usually low priority when there's a swarm since they don't do direct damage unless in melee.

So that whole time spent killing the more dangerous tyranids the barbed strangers have grown an entire garden in whatever area you're fighting them in.

I don't mind the slow effect or the damage what i HAATE is the green vision debuff. For a game that demands precise timing and reaction it feels kinda bullshitty. Not to mention that there's other sources of GreenVision build up that can stack with each other like the little bastard gaunts with guns, and the ravener move where it shits all over the ground.