P-control is only one, more simple, part of PID. PID control involves three unique variables that allow a subsystem of the robot to reach a target or maintain a target position extremely accurately. The integrated PID is best for maintaining a position but it is not designed for moving systems as one would think. This is because you can’t tune it and physics will work against you.
What this means for a tray is that you would need your own PID control loop to deposit stacks smoothly. You do not want the motor to go at 100% velocity while depositing. Though you can use the integrated PID and just decrease the velocity of the motor, the motion of the deposit will not be as smooth as a custom-tuned PID loop. You want to go near max velocity at the beginning of the deposit and slow down significantly the closer you get to the target angle (~90 degrees).
It isn’t difficult to understand how this can be an extremely powerful tool if you read up on PID loops, but for anyone confused as to why the integrated PID won’t work effectively: it’s because you can’t tune it to your specific case. The motion of the deposit will be jerky and simply not as good as a custom-tuned loop.