We tend to spend quite some time on URL management in order to get them nice, SEO friendly in all sorts of ways, but I’ve noticed that most CMS I’ve been in contact with have overseen one of the areas that I think is one of the most important ones. That is proper handling of permanent redirect (301) of URL’s when they change.
The URL structure should of course reflect the site structure, have relevant keywords in it etc. When content authors work with the site, those URL’s may change. The old ones give a “404 Not Found”. Bad from a SEO perspective but it also means current visitors on the site will have downloaded html pages with broken links. My experience is that this escalates when having integrations to other systems that generates items.