Just because it often works doesn't mean it's a good idea. Updating a single package is officially unsupported [0] and it's burned me personally on a number of occasions.
weel it's a bad idea unless you update just one package that doesn't have too many shared dependencies, things gets complicated with shared dependencies across their packages, so that's why it's better to run the full -Syu of course not automated but when you know what you are upgrading and possible manual interventions (if any) or caveats that you might have with the changes.
But sure, just yoloing a single package upgrade can break things, obviously.