I remember for a while Google Drive worked on a tags metaphor and it was just too much friction for users. The UI for the traditional file tree is just plain cleaner. Trees provide clear delineations of ownership and categorization. They suffer from the limits of hierarchies, but you can break them down and get a nice tree-view of them, for example.
Having spent time categorizing my photos, adding a tag layer on top of the existing filesystem is a great idea, but using it as a replacement for the traditional directory structure isn't.
Simple operations like "delete everything in here" becomes complicated under a tag structure now that we no longer have a strict concept of A is within B.
FWIW you can still work with Drive like this, though it's somewhat hidden functionality. "Shift+Z" after selecting an item will bring up the otherwise hidden "Add to" dialogue, allowing you to add an object to multiple parent folders/tags.
I think it's way too early to say is this kind of models good replacements of traditional file systems since we are so used to work with hierarchies everywhere and tooling is very young if any.
I think simple operations remain still quite simply but we need to alternate those a bit. For example "delete everything in here" may not be very clear thing but "delete these files globally" and "unlink these tags" are still simple concepts. What tags are offered to be unlinked for what files is just UX decision where is multiple quite ok answers. For example if we browse files as tag stack we may delete first one or ask how deeply we want clear stack tags.
I wasn't aware of the Google Drive thing; that's a shame it was found to be confusing to users. I'm glad Gmail still has tagging, because I do label my messages in multiple ways, and would find it agonizing to use a hierarchy.
Regarding photos, I fully intend to have a tag-only system to organize my own collection. I see file names and folders and counterproductive.
I agree with you that tagging becomes more complicated, and the semantics of "delete everything in here" is different. Gmail provides some insight into this - you have to distinguish between untagging a bunch of files versus actually deleting the messages (and any tags they carry). This isn't a dealbreaker in my opinion; it is a consequence of being more expressive.
But then don't you have the same probelm of the limits of taxonomies and you start thinking "well is this tag a member of A or B? It should be a member of both" and then you need to tag the tags. Turtles all the way down.
You can have preferential ordering of tags, which, basically, gives you hierarchy.
Or, put another way, the hierarchical file name can be seen as an ordered n-tuple of tags.
Both approaches give you new ways to view, find and manipulate FS content. What about viewing all files which are sources (belong to "src" directory in any part of a path)? Something like that.
Having spent time categorizing my photos, adding a tag layer on top of the existing filesystem is a great idea, but using it as a replacement for the traditional directory structure isn't.
Simple operations like "delete everything in here" becomes complicated under a tag structure now that we no longer have a strict concept of A is within B.