For this, let's ask Bill the Negav Scientist as he has recently published a paper on this matter.
Bill- Well, there are thousands of portals opening across Felarya every day.Of course, most of them close after a short time, and only half of them have anything come through. Of that half, only half of that has sentient life come through. And of those cases, of course not all come through intact. There are cases of people going insane from the trip, leaving parts of their body behind, and even coming through as a different gender!
Of course, if there was a story of someone being horribly maimed by the dimensional trip, it would be quite the short story indeed. And so, most important characters come through fine.