I found that fast as possible keywords need more maintenance than a structured keyword hierarchy does, unless I was silly enough or inexperienced enough to try getting the entire hierarchy structure fully fledged right up front. I use a basic structure and flesh it out only as needed. e.g. if a search finds much more than 30 or so irrelevant pictures to browse through then perhaps another keyword or phrase is needed in the structure; otherwise, I don't bother. Tags are quite unreliable because there is no context except other tags, and none of them cane be repeated regardless of context. There is no way to know how other users will think when looking for something, and there's no way to be sure even you will think the same way next time you look for the same thing.
Maintenance of a keyword hierarchy in LrC will be reflected automatically in the allocation to the relevant images within the same catalogue. That applies to spelling corrections, relocation to another parent group, insertion of new intermediate subgroups, etc., so there is relatively little list maintenance to affect all relevant image files.
However, I recognize that we all work differently and that some of us are trying to manage keywords for other people rather than, or as well as, for ourselves. No doubt that is why LrC has both systems available. Luckily for me, my keyword hierarchy is for me alone to use and it does not need to be stored within my image files. That avoids triggering unnecessary image file backups and allows me to exclude tags brought in from external image files. It also makes it easier to alter my own hierarchical structure without being so restricted by what is already used in out-of-context image files. That makes maintenance easy for me.