There are negative rumors about every messaging app, including session.
To be clear, there is no one tool that is guarantee privacy and anonymity. Best is to manually PQC PGP encrypt/decrypt in a offline computer and transport the encrypted files with a SD-Card, USB-stick or DVD to an online computer where you have your "secure" messaging/file transfer app.
My opinion has more optimism than most. They have shown to be open of anything brought against them and are trying their best to be good at what they do. I do not have enough brain to understand the removal of PFS but I think that for most use cases it is fine to use Session if you use it with Tor or at very least a VPN. They do have their own onion routing but there havent been any cases where it has shown to be more effective compared to Tor. Every application has flaws but look for the ones that actually try to respond to criticism.
To be clear, there is no one tool that is guarantee privacy and anonymity. Best is to manually PQC PGP encrypt/decrypt in a offline computer and transport the encrypted files with a SD-Card, USB-stick or DVD to an online computer where you have your "secure" messaging/file transfer app.