Similar to Faye, I don't think I have ever seen a step up trailer/float here unless it is a stock crate, and then you back up to a loading ramp or dirt mound to load. Never heard of an accident like that, again though nearly all ramps are factory fitted and don't have a gap.
plus the ramp gets horse poo wedged in it and you have to clean it out almost every time you try to close it.
I don't know about anyone else, but I clean out the floor of my float EVERY TIME one of the horses poops. It can rot the floor away if you leave it, and it stinks, and makes the floor slippery for the horses, and it is really bad for their feet to stand in poop. I have never had an issue with poo blocking up the hinges as I just sweep out the hinges as I am sweeping out the poop.
We have a big three horse angle load, we generally walk on and turn around and walk off, though none of our horses has an issue backing off either.
I have had plenty of horses step off the side of the ramp when learning to load/unload and as yet haven't had any injuries.
I've never used a step up so have no idea if it would be better/worse, but for my horses who are used to a ramp, it would be a big shock I think, lol!