They are no more safe than sending a message like this :). Except you would be the only person it's targeted for.
besides the admins of both instances can read them as well.
Which is why I'm the web interface it says it's not safe/e2e encrypted.
Worried about it? Add a matrix handle to your profile and then it enables a "send a secure message" button in the UI. And redirects people to use matrix to send messages to you
Nothing on lemmy is private. Your instance is just hosted on a server, and in this instance that server is essentially just someone elses computer. Anything you do or say on the server can be viewed by the admin and whoever they decide to delegate access to.
This is true for practically every online service ever.
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)
DMs on the fediverse (and Lemmy) are posts with a specific visibility that marks them as DMs. They are sent like any other posts, so there's no encryption and instance admins could in theory read them in the database.
e2e encryption is pain in the ass, key generation and exchanging, complex and annoying to do
No, no it's not.
Yes, it's more complex than sending plaintext. But for starters it'd be extremely simple to generate a keypair for every user and publish the public key with their profile. When sending DMs you'd use this public key to encrypt the message.
As for storing the private key you could encrypt it with (a derivative of) the user's password, and store it decrypted possibly just in the user's browser.
This simple measure would prevent simple ways of reading the DMs, though obviously you still need to trust that your instance admins are actually serving you the code they claim they do. But it'd definitely prevent "accidental" misuse.
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.