The Second City Saint will step inside the cell knowing that he will finally be able to get his hands on Paul Heyman, however, he will have to go through Ryback to do that.
CM Punk will have to counter the threat of two men instead of one and that, in itself, it a bit too much to ask inside the cell. Heyman could provide Ryback with all the weapons of destruction while the latter could use them to demolish Punk.
CM Punk has chosen the stipulation of the match, but many might argue that this was not one of his smartest decisions. He has decided to stack the odds against himself and that is never a good thing to do.
However, knowing Punk, he is bound to have a plan. Whether that plan is successful or not, remains to be seen.