Possum can and do prey on chickens here in the states. My dad killed one HUGE possum one year that he caught in the act of eating the breast off a roosting chicken.....she was still on the roost as he was eating her alive.
I can't imagine a GP that would let a possum live in their territory...
Pros~
Natural chicken life takes place, where they perform the behaviors that chickens out on the land normally do, foraging for the type of feed that they can best digest and utilize for nutrition, where they can seek out and consume natural anthelmintics, come and go as they please, rest in...