I can't offer a comparison with Session, since I'm not familiar w/ it. At a glance messages seem to be routed through some nodes that belong to a pool of service nodes that run some cryptocurrency stake (but I don't know what this means in practice). It does seem seem to do multi hop routing which means its more resilient to privacy attacks (but this says nothing about resiliency to being blocked).
On the SimpleX side, anyone can operate a SimpleX SMP server - that is the server that holds messages while in transit from the source to the destination (each server has a number of queues, each is one-way from a sender to a receiver ).
Each user defines the servers/queues he uses to receive messages, but not to send (those are the defined by the user you are sending messages to). So resilience to blocking means both users need to diversify the servers they use.
The folks running SimpleX host a handful of servers - and I expect those are the ones most people use. In that sense they are a point of failure for someone to block communication. If you check the source you will see an incomplete list of servers there, and in the app settings there are more (and you can add your own).
As for blocking the protocol, the following approaches seem standard for a state operator:
- block TCP port 5223
- if a different port is used, block based on TLS negotiation - this seems easy to spot
- seize the public servers
(This is as far as my knowledge of SimpleX goes - the rest is slightly hand wavy assumptions I never checked)
I don't recall how the SimpleX app manages those server queue(s?). Taking a peek at the app right I only see one receive/send queue when I select a contact. But in theory it should be possible for it to have multiple queues per contact. The documentation does mention this in some comments (newQueueMsg: maybe it is not implemented?)
Finally the android app seems to support integration with ToR and will support .onion addresses if this is enabled, that is probably the most practical way to bypass some blocker (assuming ToR is not blocked :D). But this requires that the SMP server used by your contacts supports ToR addresses.
It would definitely be nice to see support for tunneling over other protocols, and of course more servers running those (ToR, I2P, gnunet?, etc, etc).
Some links to stuff: