[kevincox]

Jami is more or less completely peer-to-peer. Other than initial bootstrapping all conversations are synced between participant devices without any central servers. I don't know much about Session, but Matrix requires a homeserver that is basically always running to operate which provides a single point of failure for any particular user (as opposed to Jami where each of a user's devices can sync independently) and is easier to use (you don't need to find and pick a homeserver, all you need is your device).

Jami has also been around a long time. It had other names in the past like SFLphone and Ring.

[bandali]

This. Jami is truly distributed, and there's no need to set up and manage any server(s) in the sense that one would do for Jitsi, Matrix, BigBlueButton, etc.

Also, thanks to Jami's distributed nature, it can also function in local networks without internet connectivity:

https://docs.jami.net/user/lan-only.html

[Yoric]

Well, there is Matrix p2p, but it's not ready for prime-time yet.

[Arathorn]

arewep2pyet.com. making progress tho!