Same weak spot as the Hidden album
Locked Folder opens with your device's screen lock. Anyone who knows your passcode, or can use your face while you're asleep, can open it. That's fine for keeping photos out of the shared grid, but it isn't a separate lock.
Backup and keys
When backup for Locked Folder is turned on, the photos are stored in your Google account. Google's general privacy protections apply, but the keys aren't held by you alone. Veilbox backups are encrypted on your iPhone before upload, using a key derived from your pattern.