But are they ? I posted a comment a couple of days ago via lemmy.world on a 'remotely hosted' community, and then viewed the same post via its 'native' instance, and my comment never showed up, even days later.
There are some issues regarding sync. If an instance is not reachable when an action is synced (e.g. due to overloaded servers), the sync is never attempted again and the instances will stay out of sync.