It’s not only the 787, but also the 747
On autopilot or on manual flight the A/T does not work and does not provide enough thrust for the aircraft to stay in the air. Speed control also fails because of this.
For example, you set it to 180 knots, but the speed keeps dropping, then the altitude drops as it tries to keep the altitude by pitching the nose up, so unless you take over the throttle and switch A/T off or the plane crashes. This happens regardless of speed by the way, it can be at 300 knots and it will still throttle back and the plane goes into a dive, not maintaining altitude.