Janssen isn't so bad that the other team will automatically score every time he's on the ice. We put worse lines on the ice last season and the team didn't spontaneously lose the game.
He's actually not terrible from a defensive standpoint, but by no means is he good. I would like to see him play the puck rather than the body from time to time, and I do think that is something he can improve upon with time. Actually, the he played, the better he did seem to get with it. Not good enough to say he deserves a spot on the roster, but good enough to realize that he CAN be utilized when playing physical teams. He won't be a terrible liability on the ice.
I hope Asham won't be a liability, either, as he is known for taking bad penalties -- think it's something he'll be able to clean up a bit by playing with the Devils and potentially having a Madden/Pando combo for linemates. With that said, he's likely destined for the fourth line though.