This is a good question!

Signal is superior because it uses a cryptographic ratchet. Right now if either party had their key hacked the entire conversation history would leak. If someone wants to propose a ratchet-like mechanism for nostr DMs that would be amazing! There's no reason we couldn't build something like signal on top of nostr.

Right now metadata is leaked, you can see who is DMing who and when (just not the content). We have a fairly simple fix for this soon which will prevent people from fetching this information from relays if you're not on of the participants of the conversation. This information is still on the relay though so it's not perfect.