Little known advantage of Nostr DMs: No one can know if the chat is complete or if there are missing messages even if the nsec leaks.

Nostr doesn't link DMs to one another in any way. Double ratchet schemes, like Signal for instance, derive keys from the state in previous messages making it a verifiable chain. If a message is missing, whoever has the main key to decrypt and derive the chat will know it if the chat is complete or not.

On Signal, or any ratchet-based technique, you can prove you have all the messages.