The simplest problem, and one I've articulated before, is that true denizens of Carceri are the kind of person who would stop right before the finish line so they could turn around to flash the middle finger to everyone else. Carceri is the plane of the monologuing villain who just has to let slip his master plan in an attempt to make the hero even infinitesimally more miserable. It's the plane of sour grapes. Why wouldn't they go after the Mercykillers' portal, or any other known and stable one? Because it would mean accepting on some level the help of others was necessary to do what you couldn't alone, and everyone else in the multiverse is a schmuck. Not only are they all schmucks, but it's also vitally important that you let them know they are all schmucks. Gods forbid someone be allowed to be wrong on the Internet think they were in any way needed, or in any way more powerful than you.
Yes, I understand, thanks!