When we used 5Ys at Uber SRE I always like to say that you are looking for a tree of depth 5 but if it's width 1 you've not tried hard enough. "Root cause" or at least the idea there was a single factor that was the defining feature of an issue is hardly ever true.

There is quite often one symptom that is pivotal and cause a critically failure, but generally that symptom itself is a result of multiple inputs.

I'd simply say 5Ys should be a rule of thumb to get folks to be sufficiently inquisitive to keep asking follow-up questions about whatever they are investigating until they've hit around 5 questions per area.