Agreeing with john's answer, I just wanted to add that other than the double/half damage for unassisted/assisted, the trajectory is totally automatic while in assisted mode, only a really lucky lineup will penetrate other zeds (although if you line up a shot normally even though assist is on, it makes no difference)
Basically if you have the time to aim (most cases you can no scope and its fine) then always use unassisted. I am not some CSGO pro that can 180 noscope people, but with some practice i don't use aim assist or even the scope.
This is also due to the time it takes to properly lock on while scoping in assisted mode, i can fire 3 shots without scope and assist in the time it takes a person to fire 1 assisted shot at half the damage. I would recommend never using assisted mode unless its a case like Patriarch is running away