It's hard to compare the two because people usually oversee that Matrix Servers, SimpleX Servers, Signal Servers know who you are to perform the right access controls into your chat rooms. While no one else knows about your messages, the servers do have a LOT of leverage over their users. Any legal action can simply target the server operator, they can turn on several tracking mechanisms without your knowledge and then metadata privacy is pretty much gone.

The goal for the GiftWrap idea is to remove the need for Nostr relays to authenticate users into chatrooms. While everyone can now see GiftWraps being received, they still can't know anything else about it. And since the GiftWrap protocol uses multiple relays to pass messages around, it is extremely hard for any legal action against the server operator to break your privacy. 

Now, of course, that all depends on compliant client implementations of the GiftWrapped DMs. Using the same protocol, a client can take the DM experience to such a privacy level (e.g. creating a new Tor session at each message to avoid IP tracking, minimizing nostr filter correlations, etc), that it becomes certainly better than Matrix, Signal or SimpleX. Enforcement wouldn't even know what to target to get your metadata.