Success messages get neglected almost as often as error ones. "Done" only confirms the action finished — it doesn't say whether the save actually reached the server, synced, or what specifically changed.
A specific success message builds trust. "Saved" is fine; "Saved — synced across all your devices" is more reassuring. When it applies, pairing it with an undo option in the same spot is even better.
The demo's ✕ just confirms it's over; ✓ tells you the result and its scope.
When to use
Add a concrete result whenever users might wonder "did that actually work?" — save, send, checkout confirmations.