If the path of the marbles were to be determined by some sort of gate in the path that would open or close depending, via a Geiger detector, on whether some uranium atom had decayed, then, as far as we know, the path would be truly unpredictable.
My mental model for this is to consider boiling water in a pot. You can make easy statements over how long it will take for all of the water to evaporate. It is, to my understanding, impossible to discuss any particular molecule or atom.
That is to say, the closer you get to the atomic behavior of atoms, the harder it is to make "deterministic" descriptions of what is going to happen.
We often describe this in terms of "coin flips" as you can make a decent discussion over what you will see on 1000 coin flips, but you cannot make such a statement over what you will see over 1.
