I find AI groups will lose cohesion in a prolonged firefight. They will break formation and as a result AI unit tends to be alone and becomes easy pickings.

Here's how it happens.
When in contact, AI group leader will issue engage command to its subordinate and the subordinates will carry out the order and break formation. Then after carrying out the order, assuming new target is detected near the target, the AI leader will issue to attack this new target to the closest friendly. Thus in prolong firefight, the subordinates will become more and more disperse and further away from the squad leader.

My proposal:
After successfully engaged an enemy, the subordinate must return formation first before the AI leader can give new engage order. This way AI wont wander off too far away from the squad leader and make them more deadly