Open editor and place your character :
Go to prone position and look around while aiming (not with freelook that only move the head up), your body will bend in a unrealistic, impossible, ugly and immersion killer way.
You can also notice that this way prevent more than 45° aim up angle.
If you use your mouse to aim left or right you will notice an unrealistic and impossible ground rotation that the character do, bot also do the same thing, their is no animation for them and depending of the rotating speed the animation is totally not matching with the rotation.