I'm certain that (again, like bullet journaling) underneath the hype is a decent productivity system, but you have to wade through hordes of breathless fans eager for you to click that subscribe button, etc.
Over time this produces something like this (https://imgur.com/6I9ojRG), which is what I meant by self-organizing. Just with back-links without having to create any hierarchies (what i meant by unstructured) you get a data-structure that is actually still pretty insightful.
And to answer the previous commenter,
> How did Zettelkast become so popular out of the sudden?
It has not become popular out of a sudden. Different applications for the slip-box method for taking notes existed long before. Roamresearch has popularized it relatively recently. I guess maybe they've done something right; I don't know, I never used Roamresearch myself.