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.
Maybe you implied it, but adding to the 'going up' case, there is bullet ricochet -- also reffered to as skipping or bouncing bullets -- that makes it so that bullets will ricochet out at a shallow angle and ride out the wall or floor, making your head hittable without even aiming at it.
Here is a instructional FBI video talking about it.[0] And a more modern take.[1]
Imagine a handgrenade- with shotgunshells and 6 cameras glued to it. It can trigger the shotgun shells, giving it the ability to "hop" on a vector. It has a primitive friend/foe system (friends are behind the plane it was launched from) all else is foe.
You throw it at the start of the stair-case it hops up, blows the door open, enters the room, finds a target, explodes near it. All in a series of bangs.
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.