FWIW, I have seen PIO routinely, in any heavy aircraft I have flown: A320/330/350, but also B737/777. The problem lies not so much in the flight control laws, but rather in the fact that some pilots will give an order, and then the opposite order without waiting for the aircraft to complete the first order. Any heavy aircraft has inertia and will take some time before executing what you request from it. Be patient…