Well from a reality standpoint, the real danger of an alien weapon isn't the impact force, but the heat. Regular armor is useless because Kevlar just melts under heat, which then melts through your poor soldiers. Nasty stuff. Jackal Armor uses Ceramic plates that Vaporize away as they dissipate the heat, hence their ability to mitigate the damage. Holding a metal shield in front of you, the metal may be able to survive the heat from one blast, and since your not making contact with the now very hot metal, your not going to burn. With enough fire, The whole thing eventually melts away or distorts significantly enough to be useless. Alien Ceramic armors and shields just fare better because alien materials handle heat so much better.
In a gameplay standpoint, I'm playing around with the one 'reasonable' shield use, which is to make your stun troops not completely suicidal without relying on gas grenades. So I made the shields two handed, and I'm about to see if giving them a stun attack causes any game crashes. If not, I'll just transition over the baton stats and say that a trooper can hold a baton behind it effectively, but the shield is impossible to aim around and keep upright. Maybe raise the weight a bit too if it is still too strong.
Holy wall of text.