I never thought I'd see the day. BI is fixing a bug that has been in this series and engine since Operation Flashpoint: the preservation of geometry on fallen or destroyed objects, like knocked over fences and trees.
Presently, intact geometric objects will provide proper visual and ballistic cover. If you stand behind a wall, bullets won't pass through (unless it's a flimsy wall, of course), and the AI cannot see you. But if the wall is knocked over, it loses all geometric properties: bullets pass through it, the AI sees through it, and it's no better than a bush. If you lay down behind a knocked over wall or tree, it's just like laying down in the middle a field. You're defenseless and exposed.
But now... BI is fixing it.
This fix was uploaded to the most recent development build. (Yes, ArmA 3 development is still ongoing.) You can see here that the AI is unaware of a player laying down behind a fallen wall. After shooting his gun, the AI is aware that there is gunfire in the area, but does not know where it came from. The player has to roll out of cover and engage the AI before the AI knows where to look. This should result in far less frustration with this game, because you can now approach firefights and combat more realistically.
Additionally, you can now rest your weapon on these same fallen objects.
You can also hide from AI in logical ways:
This fix is planned to ship in ArmA 3 1.83. No release date or ETA. Probably a few weeks to a couple of months.