I've spent some time trolling through the tag system (been bored) and updating any blank entries I might be a fan of. Found myself eyeing the Mistborn section and trying to presume "how it should be working" with the author tag, the series tag, the subseries tag, etc.... led me to search and find this question, which seems to be the most recent and more comprehensive at documenting multiple tag issues.
Unfortunately that question didn't seem to lead to any sort of consensus. There are two differing answers, each with the same upvote count, one that looks like its advocating a barebones one-tag approach and the other advocating tags and sub-tags (at most, limiting author tags). Everything else I see about tags appears to be a lot of single-use questions asking if something should be removed.
I would like to continue helping make the tag system useful where I can (or at least somewhat, as I think "close enough" is best that can be hoped categorizing for a fanbase as varied and opinionated as most SFF fans are). Since I can see the reasoning behind both approaches, my general takeaway from the non-consensus seems to be "keep it simple stupid by default, unless you see the demand is there for more". Applying this to the Mistborn examples wouldn't lead to much changing (all the tags are well in use), but might guide some cleanup (mistborn tag clarified to apply to entire setting, subseries tags removed from questions that aren't novel specific, maybe clarifying the subseries tags (wax-wayne-novels is an awful name) etc.)
tl;dr hierarchy seems unavoidable in the long run, but the more we can trim and clarify the better. Is this the best approach?