Arathorn

* In the current Matrix implementation, server tells the clients what the group membership is, so a server could spoof an invite to add a malicious user to a room and you'd have to rely on social mechanisms to spot it.

* There's no way to force everyone to rotate the sender key at the same time.

* On the plus side, it decentralises relatively well.

The concrete disadvantages of MLS are:

* Vanilla MLS needs a centralised sequencing function for changing group membership.

* The key bundles are much larger.

* On the plus side, only clients within the group can add new users/devices, and everyone in a given epoch shares the same group secret, so when someone rotates everyone has to rotate to participate in the new epoch.

Now, the server-controlled membership problem can be solved with Olm/Megolm if you instead have the client control the membership, using Matrix itself to define which users are authorised to be in the room, by running the Matrix DAG merge resolution on the client as well as the server. I did this a few weeks ago (in prep for The Matrix Conference next week) to show viability; it works, and has the advantage that you don't have to blow up the whole crypto implementation.

Speaking as the CTO at Element (and proj lead for Matrix), i feel like i've spent a bit too much time coming to chat about Matrix on HN over the years ;)

That said, I agree that Bluesky is doing well in terms of breaking through into mainstream awareness - much more so than Matrix (although obviously big-world-social-media is something of a different beast to distributed-instant-messaging).