Don't forget that ordinary damage also contributes to stunning.
I've had three orlocks shrug off a gas grenade, but followed it up from a shot from a plasma-armed grenade launcher. They became stunned rather than killed.
I don't know if the stun rather than kill in this instance is certain, or based on the damage delivered by each, or what.
But it is still a particularly useful thing to know when your stun-rodder has just run up, zapped an alien, and the alien is still standing. Get another soldier to finish it of with with a conventional weapon and there is a very good chance it will fall stunned rather than dead.