Operations you tend to perform on tags: "find me posts that have all of these tags", "find me posts that have similar tags to this one", "order the results of any of these queries by properties of the post". Something you usually do not do with tags: "give me a list of every tag that the system has ever seen used on a post". Tags normally don't themselves have properties. The event of a tagging usually does not (but sometimes does) have properties either. I am not at all certain why you would find yourself needing to UNNEST: there's nothing you'd do with a tag where that would be interesting or valuable.

In my previous comments, I offered multiple "practical advantages" of using arrays for this case over relations; you are choosing to view them as "microoptimizations", but I'd say the exact opposite: every operation one normally does with tags, when used with arrays, is a single operator in a where clause. Even the simplest case, "show me posts with these tags" is at least slightly harder with the relation. In general, the clearest code with the right semantics tends to also be the one that gives the database the most room to optimize: its the same here; you are just muddling up the N-dimensionality of tags by moving them into a relation.

Your attempt to compare a list of locations doesn't make much sense to me, as a list of locations doesn't seem to fit well with the question "give me a list of posts that have similar sets of locations to this other sets of locations"; if you can explain the semantics of that use case more, I might be able to appreciate it. However, right now, I think you are still just caught up looking at tags at the wrong scale: a set of locations is more similar to a set of tag clouds, not a single tag cloud; a tag cloud is a single location in an N-dimensional space, and all of the operations developers tend to perform on that data type are easier to think about, easier to optimize, and even easier to type the SQL for with PostgreSQL, than thinking about them as discrete data items.

What separates PostgreSQL from other database systems in this regard is that they actually have index implementations that can make this obvious translation of the semantics of tags fast: normally, you are forced to do the relation, and you end up in the hell of working with relations for things where they make no sense at all. As an example: "give me all the posts from last friday, and all of the tags on each"; this is the exact opposite of your UNNEST scenario: you end up having to rebuild the records of the posts here by doing a join and an array_agg. It would be ok if you bought something, but you didn't, and the only reason it even seems to be "more relational" is due to a backwards understanding of what a tag cloud is (like, to be clear: I really don't like the denormalization-is-good crowd, and I think NoSQL is generally the wrong solution to any problem, but I maintain that splitting a tag cloud into a relation is like splitting a basic data type like a long/lat, or attempting to do an ORM columns->rows mapping, both of which misapply relations; its like you are reading what I said as if I'm talking about long/lat being changed into an array, and totally ignoring the semantics of a long/lat as a location in a two-dimensional space... the fact that "PostgreSQL already has yo covered" for something like long/lat precisely demonstrates that people wouldn't even consider breaking those apart).

Can you provide me an example of your problem using arrays here? You seem to just be asserting the same point that they are somehow harder over and over again. The only such example I'm seeing in this post is "uniqueness guarantee", which is a property of a single array and thereby trivially described as a check constraint on the column. This kind of set-oriented use case, FWIW, is actually precisely why GiST and GIN were created, and why they support the intarray contrib module; and this kind of facet-oriented search is one of the reasons why PostgreSQL got so many great contributors via PostGIS.