They suck all around. Going down, your legs are exposed, then your chest, then you finally have the ability to see the next floor, so your gun and your eyes don't get down fast enough to be able to process the bad guys in time before you get filled with holes.

Going up, your head is exposed, so that becomes the primary target, when people typically aim for center mass. But center mass isn't visible immediately, so head takes all of it. On top of that, the whole 360 degrees of the next floor becomes visible at once, so there's no way to take slices of the room at a time, like you would a doorway.

Staircases are very brutal. But really, room clearing as a whole is the last thing you want to be doing. You can do it completely correct, and still die. Room clearing is a matter of minimizing chances, not eliminating them.