There is a gap between “everything works” (title) and “nothing works” (what you read).
I’d say the issue is more about reliability, this comment expresses it well:
I try to stay away from cloud integrations and cloud-based stuff like the app itself, so mostly use automations and Matter devices, they’re local and pretty stable. But there’s been indeed episodes every few months when something breaks, so I always remember that phrase from @JDRoberts.
Then there’s the missing features, would you consider not being able to do something as “doesn’t work”? I enjoy squeezing the features of Matter devices but not even a simple “increase brightness” action is available in SmartThings unless you make your own custom driver. Same goes for light fading (transitions). Probably not a deal-breaker for many, if I didn’t know how to make drivers I would probably be using HA already which has those features, but even HA has its issues too so… 