The most important difference is in design:

• many ceramic packages have a cavity etched into the upper ceramic slab to accommodate the lid over the chip.
• Between the lid and the die is an air space — a relatively gigantic gap across which ultrasound will not pass. Scanning the ultrasonic transducer above the topside cannot image such a package. The solution is to flip the ceramic package over and image it from the backside.
• If the package has already been mounted on a board, the die region is inaccessible from either side.
• Plastic packages lack this gap, so a board-mounted plastic package can still be imaged nondestructively. In this respect, the switch from ceramic to plastic has an extra payoff — plastic packages are easier to image acoustically.