Also while in the managed descent mode the aircraft has a 20 knot margin either side of the planned descent speed. The descent is planned at thrust idle. If it gets high on the profile it will increase the speed (and descent rate) to regain the profile. If it reaches the upper margin and is still too high you will get a message asking you to extend the speed brake. If it gets low on the profile it reduces the speed (and descent rate) to maintain the profile. If the speed reduces to the lower margin and the aircraft is still too low on the profile it will add thrust automatically.
Descent in the open descent mode will reduce the thrust to idle and the planned speed will be maintained with no margins either side. The aircraft could then end up either high or low on the profile. While you are still able to determine whether you are high or low, the aircraft will not automatically fix it.