I went into the queues last night with my new sci and could see the root effect of the Autocarbine in action for the first time. It prevented me to move and turn for about 2 seconds at most, and all I could do was crouch. Although the root duration was small, it was plenty for the enemies to flank me to death.
My sci is still away from omega sets with root resistance but he does have Willpower. I noticed the effect mostly when two or three enemies were attacking me at once, but I was not able to tell at that time if more than one had the Autocarbine and/or Suppressing Fire active.
On the other hand I've rarely (if at all, I honestly don't remember) seen that root effect on my Engineer. I guess it comes with willpower + omega sets with root resistance (and maybe traits, he has Sure Footed as well), and I'm hoping it will be the case with my sci when he gets them.