> plenty of fast native clients

And God help you making all of them talk to each other properly...

I have used conversations (and derivatives), gajim, dino, converse.js, and movim to have omemo encrypted xmpp conversations; and I talk to people on Siskin and have used profanity in the past. Seems to work properly, unless I'm missing something.

Before XEP-0459, every client implement their own subset of standard and seldom talks with clients that implement a differnt subset.

Now try to make a voice and/or video call.

I do/have, with conversations, siskin, movim, and I think dino. Can you expand on your experiences?

gajim, pidgin, dino... none of them worked when I tried to call from my desktop to my Conversations' app on Android.

Does any Matrix client support voice/video calls other than Element (including Hydrogen as it shares Element Web's codebase)?

Fluffychat supports it too. Check out https://matrix.org/ecosystem/clients/, you can click the clients for a feature list.

Hydrogen does not share a single line of Element Web’s codebase.

I must have got it confused with another client then. Wasn't there another lightweight one made by Element-the-company that reuses the react SDK?

nope.

> - simplicity of the protocol, especially the encryption

only simple for developers though - due to the lack of Cross Signing it is very difficult to keep communication secure.

What is cross siging in the messaging context?

implicitly verifying all (including future) of somebody's devices although running through the verification process only once.

Without it, e.g. if you log into a new session, you have to verify with all your contacts before you can safely use that session for communication