Why nested tags are not that good?
One common pattern I frequently see is the one with tags being nested in a hierarchy. It might be called as nested tags as well.
They are like this #thing/subthing. And in theory, one could create categories and subcategories.
The thing is that with time, some arbitrary “categories” might make sense as if they were #subthing/thing instead of #thing/subthing. Or you might have #other/thing or #other/subthing.
— There's no issue, you say, I simply create those and move on.
Yes, it is possible, but if you want to query / search for the tag subthing, for example, you can't. You'll have to query / search for the full hierarchy, making it more complex.
Now, if you use individual “flat” tags such as #thing, #subthing, and #other, then you can query individual tags regardless of their combination and by requiring that two tags be present, you can query any specific case where you would have used nested tags.
Some example cases:
#medicine/drugs,#studies/medicine#work/travel,#personal/travel#work/meetings,#church/meetings,#family/meetings
These all demonstrate possible hierarchies where you might want to see multiple things (e.g. all travel related notes) and hierarchies /nesting cause issues.
Having flat tags imply automatically on more combinations of tags and allow for more flexibility on your vault.
But…
As always, if you always plan things in advance and review the available tags, you can use nested tags without issues.
It will make it harder performing searches, but you might see that the benefits are greater than the trouble of such queries.
As my workflow relies heavily on using search, these impact me all the time.
As with anything, you have to assess your use cases and document your vault taxonomy to prevent creating multiple permutations of the same hierarchy by mistake.