I think that given the descentralized aspect of the relays you may always be exposed to the “delete” message not to hit all relays where your message is stored
Let’s say you have a message on relay1, 2 and 3 and after some time you change relay 2 and 3 with other relays. If you send the delete message it won’t delete it from those two and people will continue to see them
It can also happens that certain relays don’t apply that and keep everything so you may need to chose which relay to connect if that’s a key attribute for you
And finally.. anybody with your message could have downloaded it and would have proofs that you send and signed that message
So in the end delete won’t give you lot of guarantees. If you speak in public, and you say something out loud.. you can’t undo that. You can only say sorry