Not quite, let's say you're up against something like an IG superheavy tank co, or a Nid Dominatrix with the little guarding guys (who are still war engines). With a pair of cobras you move up and shoot, and let's be optimistic and say both hit. Both of those hits (which are D6 TK) go on one of the war engines (which can take up to 3 hits before one has to be assigned to one of the others). So let's say you do the average number of wounds, which would be 7. Each of the war engines (other than the Dom) has 3 wounds each, so one warmachine dies, but the other wounds don't get carried across, so are lost. Comparatively two Cobras have the chance of destroying two war engines, although this is of course dice permitting. Due to the flexibility of two, even if you say do 1 or 2 wounds to one of them, you can then re-align to hit another clean war engine. Only really an issue for Cobras, as the issue is down to multi-wound weapons. Hopefully they might be allowed as single units (but non core obviously) in the future
This is also why you tend to see shadowswords as individuals rather than a unit of 3, as they struggle somewhat against units of warmachines (especially as many are 3 wounds, which is pretty much the worst case scenario for them, with something big like a Titan being best case - if you can get a bead on it anyway!)
As to what i'm waiting for at the moment, Warp Hunters and Hornets, although i think i've put off painting some of the other bits due to lack of the final models to complete the army.