The processing time estimates on the USCIS website seem to change constantly and sometimes jump by several months overnight, so does anyone actually find those timelines useful or accurate, or are they more of a rough guess that shouldn’t be taken too seriously?

Most people don’t find those timelines very accurate, they’re more like broad estimates than something you can rely on day to day. USCIS updates those numbers based on averages and internal data, so they can jump around a lot without reflecting what’s actually happening to individual cases. They’re useful only as a very rough reference, but taken too seriously they usually just add stress instead of clarity.