Vaults stored in Dropbox could be brute forced if an attacker ever gains access to the ciphertext. The 1P service mitigates this by adding a random key as salt that’s stored locally (iOS keychain/browser storage). That key never leaves your device (authentication is done via zero knowledge proof), so the master password is virtually impossible to brute force right now, even with a very weak master password.

I can highly recommend reading the white paper, it’s very well written and kinda like a comprehensive guide to E2EE which every SWE should be familiar with anyway.

And what do you mean by Apple being lethargic? They added a secure enclave to all their devices that is probably the most secure storage and crypto processor you can hope to get for that kind of money. They also added the option of completely end to end encrypted backups. Because of the secure enclave, that’s actually a really safe option.