To test this create a character with basic movement in the character bp, set the friction to 0 and the acceleration low (512) now ingame accelerate forward and when you reach max speed let go of all the keys, the character stops very suddenly (doesnt seem like 0 friction to me) now accelerate again but this time when you reach max speed switch from going forward to going back (holding w switch quickly to holding s) now the character slides as if there is no friction and is trying to run the other way.
If the friction is 0 then if the character would stop accelerating surely it would not lose speed all of a sudden and keep on going at the same speed for ages.