Pros:
-Their Shots wound marines on 3+, with no armour save (except termies/artificer armour)
-They're very fast (jump infantry, fleet, re-roll failed dangerous terrain tests)
-They have a respectable T4
-Leadership 9 with a strain leader, and while he's alive they can benefit from markerlights.
-High initiative, striking before marines.
Cons:
-Paper-thin 5+ armour
-Don't fare well in assault (despite being designed for it...we'll touch on this in a minute)
-Their guns are only assault 1 and will likely only get to shoot once.
- Ballistic skill 3 largely wastes that one shot
-Monstrously expensive, costing 70pts for four of them
-Usefulness only decreases further when their target is in cover
-Weapon skill 3 makes them vulnerable in assault.
-If wound allocation kills the strain leader, you can no longer benefit from markerlight hits and only have Leadership 6
Now, vespid seem to be designed to target a marine squad with their neutron blasters then follow up with an assault, but there are several problems with this.
-Their guns, even with a full squad of vespid, will average 4.5 marine kills. 2.25 if they're in cover (they will be)
-For them to be able to assault after, they have to be 6"> away from the target, and even with their 12" movement, means they started the turn 18" away. That means the marines would have been shooting at them the turn before. Assuming a Flamer/ML loadout, that's 8 bolter shots and a frag missile, about 8 hits, about 4 dead vespid. Even fewer dead marines.
-If they were 24" away and zoomed in to 12", they would still have had a round of shooting from 24" from the marines, and would end their turn without being able to assault, but with rapid fire and flamer range. Or, the marines could fire pistols and charge.
-A full unit of 11 vespid charging marines is 22 attacks (assuming none died) 11 hits, 3.6 wounds, one dead marines. A unit of 8 marines and a PF sergeant (because one died) gets 9 attacks, 6 hits, 2.5 dead vespid (accounting for two attacks being PF, of course).
-Vespid can't fleet and shoot in the same turn.
-If the marines assault, you kill half a marine, and they kill 5 vespid (account for .5 marines being dead). Without them shooting.
And, most importantly
-Since the marines WILL be in cover, they will get a 4+ save, and you will strike second (if you assault).
Now, in literally ideal circumstances:
Marines are out of cover and you have markerlight support. 11 shots at BS5
Shots x To hit chance x To wound chance x To penetrate armour chance
11 x5/6 x4/6 x6/6 = 6 marines dead, assault means one more marine dead, the three remaining marines only kill 1.5 vespid (accounting for PF sarge) Then you lose combat by one, stick around anyways and eventually wear the unit down. You will lose a lot of vespid over a very protracted, but under ideal circumstances with and ideal target who is poorly positioned didn't shoot at you and with outside help from markerlights, vespid can kill a unit and make back most of their points, but not any other unit in the game.
By the way, full unit costs 182, more than a transport-less tac squad (usually)
I'd leave them at home...always.
