What kind of data isn't included? Are you sure the sync was active and the account was properly set up to back up ALL items?
Bear in mind that for back up, your actual app data isn't saved (just basically a list of installed apps to re-install and general phone settings). Secure Folder data needs additional steps to back up and restore (for security reasons). Encrypted messages cannot be backed up (some MMS/groupchats fall into this category). Non-camera folders usually need to be manually selected for back up.
Especially when doing a SmartSwitch transfer it'll tell you which items are not going to be possible to port over, but it may be a little more obscure when just using Samsung Cloud back up.