I thought about it again.
I forgot that he can turn arround 360 while being in the sandbag and that the sandbag is a 360° defens.
I also played a bit RA2 YR again. Now I think he shouldn't be able to move arround, but he should be able to turn arround 360° at his position. The sandbag should have a circumferance and be lower. You should be able to see the GI's body and head. Now if you give him a new armour so he is uncrushable, get a health and armour increase and enemy units don't cause much damage. His MG will have more ammo than (for exmaple) the Recrut has. It will fire faster, have a longer range and does more damage. I think that will counterbalance it. He shouldn't be able to duck. That Tanya, a SEAL, a sniper, a attack dog or a Chronolegionere still can own him would correspond to the RA2 YR balancing.