There is in fact a middle way between structured and unstructured. An example would be if you were building a contact database and when it came to meeting extended family at holiday times and asked them for their phone number, you also noted down their parents names. You could even record there children's names and more but if you only recorded their parents names this would be fine. What then happens is over time as you speak to each member of the family and get their parents name the family tree hierarchy simply "emerges" from the details.
You can see here that in the above example we have established that a hierarchy exists in the real world and ensure we simply collect enough information each time we talk to someone "Their parents" that the hierarchy builds over time. Such hierarchies need to tolerate missing information, but they can actually help us discover what information is missing, Which we can then seek.
There are plenty of hierarchies that exist in the real world that almost need not be stated like family trees and
earth > Country > state > county > town > street > number
If one assumes these exist in the first place, it informs us of what it takes to get a full address, but a fuzzy hierarchy and tolerance for missing information. for example you may only record a state/town for where a cousin lives, you can assume the planet, country and county and perhaps for now live without knowing street and number.
The thing is by being aware of hierarchies that exist or you discover, and accounting for there existence, but not "slavishly" trying to build them, these hierarchies' just emerge from the shadows over time. In many ways this helps the unstructured data trend towards more complete information over time.
To me this is where an unstructured database can exist, in such a way that overtime, the obvious, but even hidden structures start to emerge. And you see here there is not problem having both at once. In fact within our unstructured database there will be other emerging structures like lists, tables, networks and common attributes or values. For example, if someone has the "same home phone number" (land line) as another person, perhaps they live at the same address? We may learn they live together, even although we don't have their address (however we have the phone number which we can and ask for the address).
This ability for tiddlywiki to accommodate the unstructured through to multiple and incomplete structures is, I believe, one of tiddlywiki's key attributes that can empower its application universally.