This failure mode in particular was deemed exceedingly unlikely by Boeing, which got them an exception to some initial airworthiness issues with the RAT, which in turn would have made a total loss of power catastrophic.
They can deem things unlikely? That seems broken in general. I would deem it unlikely they'd ship with any errors they didn't deem unlikely; those are precisely the failure modes we should most look for...
The entire aircraft is an electro/mechanical system with many thousands of things that could go wrong, but are deemed unlikely. All engines could fail at the same time, but it's deemed unlikely. Redundant hydraulic systems could fail together, but it's deemed unlikely. There is no certainty in systems this complicated.
