Using JS to present data moves the work of generating the display of that data from the build process, as with "pure" static site generation, or from server-side code, as with a standard CMS, to the client, where it is most likely to fail in unpredictable ways due to variances in browser JS or CSS engines, network performance, browser extensions, etc, etc.

But given that the concept of progressive enhancement seems to have been completely lost on the latest generation of web developers, who cares, right?