As with all drones the Phantom balances its weight evenly across all its props.
In the Phantom’s case being a quad copter one pair rotors spin clockwise and the other pair counter clockwise. When the drone needs to turn one pair is slowed down and the other sped up. The prop yaw then turns the drone.
This means it is imperative that one pair will have power (speed) available in reserve.
If the drone is in forward flight the two back rotors speed up and to maintain altitude the front two slow down slightly tilting the drone forward.
All the while the four motors are sharing the load near evenly because of how the drone is balanced.
Adding any additional load to the drone should therefore be done in s manner as to keep the balance so that no motor or motors are over worked disproportionally to the others.
It is for this reason that Gannet Drone Fishing placed the load point under the gimbal slightly forward. This is so that when the drone is in forward flight the load is shared more evenly.
Above the load is moved halfway up on the rear landing legs as seen on other options. The back two motors are overloaded due to the off centre loading, this will affect their life at the bare minimum but much worse than this the entire drone is thrown off balance.
If the load is lowered to the bottom of the back two legs the condition is worsened as the drone tilts forward. And on top of this the downward sensors can be affected.
The above is only in ideal conditions, should there be any wind the drone would be destabilised even more.
Making balance even more important.
In conclusion, if any load is added to the drone, no matter the weight it is better to maintain the drone’s balance as near as possible than to risk the drone itself.