What problems were fixed? The limitations on the README were helpful but I wish it also included what issues were successfully addressed that prior DM implementation didn’t have 

These are the current limitations from the Readme:

“On its own, messages sent using this scheme has a number of important shortcomings:

No deniability: it is possible to prove an event was signed by a particular key
No forward secrecy: when a key is compromised, it is possible to decrypt all previous conversations
No post-compromise security: when a key is compromised, it is possible to decrypt all future conversations
No post-quantum security: a powerful quantum computer would be able to decrypt the messages
IP address leak: user IP may be seen by relays and all intermediaries between user and relay
Date leak: created_at is public, since it is a part of NIP 01 event
Limited message size leak: padding only partially obscures true message length
No attachments: they are not supported”