Not safe at all because it was not designed to be, it even tells you as much. Use them to exchange e2ee contacts and then use that.

• It is not unsafe.
• It is not 100% private. Admins can read your messages if they choose to investigate your messages.
• It will not get blasted out to the whole fediverse; just to the recipient you indicated. (Unless an admin from the previous point reads your message and publishes it publicly on the fediverse)
• You do not get to do anything naughty with it; expect to be caught if you break the rules.

DMs are basically comments without a post and only one regular user can see it. No encryption, and it can be viewed by admins. (Private message reporting is a thing)