I have to admit, that the AI quite often run into the way of my shots and block them from reaching the enemy. But frankly, that's not such a huge problem most of the time

... everybody runs in front of everybody's line of fire.

Fire discipline and target distribution is one of the most lacking yet fundamental skill in any coop shooter. 

Oftentimes tho the issue is even more  basic, when you have players that vacuum and hoard anything they find on the ground, be it ammo meds or relics, as if they could bring them home once the mission ends.