Most of my family use my brother's Matrix server, but it's the Element app that makes it appealing to us all. Client side scanning could be enforced in the app, regardless of the server's protections.
However there is a whole ecosystem of clients, and they can't all be back doored. You are also free with write your own client, and many do (which is why we have so many in the first place).
Potentially, yes. But that's where Signal's protocol helps. It includes plausible deniability.
Therefore even if your chat log is leaked by one member, it isn't possible to probe person A sent the message. If person B was the leaker, anyone that person B has ever communicated with on Signal could have sent the message appearing to be from person A.
Not in the Matrix ecosystem. The protocol is so brittle there's only one real server and one real client, probably intentional, since the designers of the protocol make money from that server and that client.
The designers of matrix decided to shift more of the burden of the protocol to the server so that clients would be easier to implement. Therefore there are few servers and many clients. e2ee makes the client more complicated, that's why not all clients support e2ee
I love Matrix but they do have a bit of a monoculture problem which I hope will get better when the protocol stabilizes. As long as they document their standards we should be fine. Their big commercialization push could turn out to be problematic in the long run, but we have to give them the benefit of the doubt and see.
If you want all the voip bits in place and all the latest features, you have to run a specific combination of synapse, sliding-proxy and element as the client. The xmpp ecosystem has similar problems but it gives a bit more leeway with various combinations of servers and clients that work well.
Matrix has more focus on IRC-like rooms, has a lot more features for that purpose and is much nicer to use than any conference xmpp extension.
