@Intrepid Prodigy
No, the cain is not more ammo efficient. The cain only fires when at 100% which won't happen often if you plan on using it a lot, and for a weapon that consumes so much ammo, I would expect it to at least knock all YMIRs into health.
But that is NOT what being ammo efficient means. Ammo efficiency is the amount of damage you can get per unit of ammo. For example, if you can kill a target in two shots with one weapon, while another weapon needs three shots to kill the same target, the former is more ammo efficient because you saved yourself one shot.
Thus talking about ammo effeciency, there is no equal to the Cain single targets.
I'm not even talking about the immense AoE of the Cain. It's so big that you can hit two Ymir mechs, or EVEN ALL THREE during the smuggling depot assignment, dropping all three to 1/4 armor (all assuming Insanity; number crunching and power gaming only matters at that difficulty anyway). You've essentially done the same amount of damage 48 grenades would've done in a matter of 4 seconds, for only the equivalent of 10 grenade ammo. Hows that for ammo efficiency?
And seriously, why would you ever use your heavy weapons against 'trash' enemies, when conventional weapons are as effective and their ammo clips are more plentiful? It's like using a sledgehammer to swat a fly.
The grenade launcher on the other hand can be replenished rather easily, will obviously still fire even when not at full capacity, and has a great amount of damage.
Again, that does not mean that's ammo efficient. Who really cares if the Cain can't fire at full capacity? You only really need to fire it against really tough fights or where burst dps is needed. Those are rare enough that you should be able to fire off 2 Cain shots (assuming you took 6 heavy weapon upgrade) when you need to;
And for the record, each regular power cell pack replenishes 2 grenades. It also replenishes at least 20% of the Cain. So no, the grenade launcher does NOT replenish easier than the Cain; they get the same amount per power cell.
I would rather use the grenade launcher or blackstorm in most situations vs. the cain, regardless of my squad-mates helping me (which is what they're actually supposed to do anyway).
As someone else has said, the situations where you would use grenades or blackstorm could be easily accomplished without them, even in Insanity (heck, blackstorm is useless in insanity due to protection). For situations where you REALLY need a heavy weapon, there is no equal to the Cain.