Your card data is private to your account — locked down by database rules so no other user can see it.
Login usernames/passwords you store are encrypted on your device before they are sent to the cloud. We can't read them, and neither can anyone who might access the database.
CSV exports leave out saved login credentials unless you check Include login credentials next to Download Blank Ledger.
Want a clean wipe? Import a blank CSV to clear your cards. There is no reset button, so this is the intentional way to start fresh.
New device tip: your encryption key lives on this device, so previously-saved login secrets won't decrypt on a new one. Re-enter them once and they re-encrypt locally.