Mentioned above several time but:
-Anyone logging with any npub can see the DM history of that npub.
-DM content is encrypted, but you can see when and how many messages were exchanged.
-Don’t use nostr for DMs, use SimpleX.
Discussion
Simplex has a really poor user experience and doesn't also allow sharing of chats across devices. I'd trade the visibility of metadata over those inconveniences.