Actually, I think that for the US general election, there is remarkably little uncertainty in some important aspects.
I mean, yeah, the whole “maybe Biden will step down thing” means that we have potentially an unusually short campaign season and an unknown replacement (though with some pretty good guesses as to who it might be). And Trump being assassinated obviously would have had an impact.
But we also have a an unusual situation in that we presently have two candidates who have both spent four years serving as President running against each other. That is, these guys are pretty much known factors. We know how they act, what they do, when they’re in office, because we’ve watched them actually do it.
Usually at least one of the candidates hasn’t been President before.