The passcode problem
The Hidden album is locked with Face ID or your device passcode. That's a reasonable default, but it means anyone who knows your passcode, or can watch you enter it, can open the album. For many people that's exactly the person they want to keep out.
Veilbox uses a separate pattern that has nothing to do with your phone's lock. Knowing the passcode gets someone onto your home screen, and that's all.
Where your photos actually live
Hidden photos are still part of your Photos library. They sync through iCloud Photos to every device on the account, including a shared family iPad or Mac. Unless Advanced Data Protection is turned on, iCloud Photos uses Apple's standard data protection, which means Apple holds the encryption keys.
Veilbox files are encrypted on your device with a key only your pattern can produce, whether or not you use our encrypted backup.
The honest take
If your main concern is a stranger scrolling past a photo, the Hidden album may be all you need. If your concern is someone who already has access to your phone, it isn't designed for that. Read more in Hidden album vs. a photo vault.