Session was at first a fork of Signal without usernames.
Now by design it uses their own custom tor-like service (instead of just... using tor) and does not support forward secrecy or deniable authentication, so anyone who collects the messages in transit can either find a vulnerability in the encryption scheme, or spend enough GPU resources to crack it, and they have confirmation of who sent and received the message and what the contents of the message are. And is headquartered in Australia, which is 5EYES and much more against encryption than the US. Oh, and the server is closed-source.
Regarding Australia's 2018 bill...
The Australian Parliament passed a contentious encryption bill on Thursday to require technology companies to provide law enforcement and security agencies with access to encrypted communications. Privacy advocates, technology companies and other businesses had strongly opposed the bill, but Prime Minister Scott Morrison’s government said it was needed to thwart criminals and terrorists who use encrypted messaging programs to communicate.
Regarding the 'vulnerability or cracking them later' bit...
Messages that are sent to you are actually sent to your swarm. The messages are temporarily stored on multiple Service Nodes within the swarm to provide redundancy. Once your device picks up the messages from the swarm, they are automatically deleted from the Service Nodes that were temporarily storing them.
From Session's own FAQ:
Session clients do not act as nodes on the network, and do not relay or store messages for the network. Session’s network architecture is closer to a client-server model, where the Session application acts as the client and the Service Node swarm acts as the server. Session’s client-server architecture allows for easier asynchronous messaging (messaging when one party is offline) and onion routing-based IP address obfuscation, relative to peer-to-peer network architectures.