The only place I believe the ball can fall through the playfield is near the Hulk bridge mechanism when the bridge is up. This is not a problem on Pro machines (bridge does not move) and Stern quickly fixed the problem on Prem/LE with a new rubber or post or something. It never happens on mine.
Overall a wonderful game to play. IM is the toughest shot for me and agree that hitting BW repeatedly when you get on a roll feels GREAT!