Fatebringer is by far the single most used weapon in the raid for thralls from what I've seen. There is a reason most of the videos of people soloing the raid use it. Unlike a scout it is one of the few weapons you can fire from the hip at full speed and get headshots with. A single headshot can and will take out the entire pack of thralls following you. A non-headshot will still get 2-3. I am with Strange and djskribbles on this one. Rate of fire isn't important when you only need to shoot 2-3 times every thrall spawn to kill all of the thralls that spawn. I also don't get the complete stop thing. Most of the time I am firing from the hip, and do that without stopping. Even when I'm not firing from the hip I run at full speed. Hand cannons have a wider field of view and so are much easier to fire with on the move than scouts when scoping (scouts generally have a higher zoom, but that results in less FoV). I would say for mobility hand cannons and shotguns are king. The only real downside to a hand cannon is reload - but on that count Vex isn't much better. You would be better off with a large clip auto. I cannot think of a single reason to use Vex over Fatebringer for the first part.
Vex is more used for thralls, and thats with even far less people owning Vex. Look around at the start area of your next raid. The next most common will be fatebringer, for people who dont have vex, and just think the overrated firefly perk is cool, not because of weapon quality.
Vex has the agility +2 as well which is even more of a bonus here.
Fatebringer is just extremely slow to handle, and it feels like you must come to a full stop to use it. I switched from Fatebringer to any old scout even before I discovered how good vex was here. I would rather use a kinetic scout than Fatebringer in that section. The rate of fire of Fatebringer is also slow of course.
It's just much better to be able to turn and hold down the fire button and know you're firing a continuous beam of plasma that "mows" than come to a complete and total halt, to line up a head shot with Fatebringer imo.
I think I'm going to do some damage testing soon to put some hard numbers on things...I'm kinda doubting fatebringer hits much if any harder than my Badger CCL.