In terms of tech and implementation details, it’s been years since everyone has been converging towards the same WebRTC architecture (with everyone bundling/linking the same set of basic components and libs as found in chrome, android, …). As such, a call between two participants (or as a group with less than a dozen participants) should be as good on XMPP as anywhere else (including the commercial options like Google Meet, Zoom, Matrix, …).

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Of course there are caveats like relying on TURN where direct connection is impossible, but that’s the gist of it. Regarding XMPP group calls,

Where things start getting spicier is in large group calls (dozens of participants or more) requiring the stream to be brokered by a central server (SFU), with stream re-compression and optimisation. Standard-XMPP isn’t great for that yet (non-standard XMPP, like Jitsi, on which it is based, is pretty damn good, but unavailable from your regular XMPP setup). Work is going on to improve that (on two fronts, with some XMPP servers turning into SFUs, and with a protocol being designed for offloading AV streams to any willing existing SFU).

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The problem with large group calls essentially boils down to how much bandwidth and CPU you want to throw at it, and that’s not cheap (unless, of course, you are the product, i.e. Google Meet, Discord & al). The same applies to self-hosted Matrix/Galene/Jitsi: you probably won’t want to hold a large conference call on a home-server, and the server admins are bearing some costs, so get to know them and how sustainable that is. In the case of Matrix.org, it is not.

No idea what prose is.

Prose is an open-source XMPP client with a focus on large rooms/banquet-style conversations (like IRC, slack, …). It is still in its early stages but already quite usable and possibly a good fit for a subset of Skype refugees.