> Tag search is a search. Can be broken. Can be optimized in a way that causes it to lie. I look at the results, and I'm not sure if they're complete. Maybe the file I'm looking for is really not there, or maybe the search gave up too early. Or the tag was slightly misformatted?

Google Docs, while not exactly a tag system, wants you to search instead of use a hierarchy, and so is my go-to example: A good 90+% of the time, I can't find something I know is there and have to ask a co-worker for a link.

The missing consideration when it comes to tags is simple discoverability. You have to already know enough about what you're looking for in order to find it. A hierarchical system lets you do systematic browsing.