Comment by asherkin

The originating browser is responsible for encoding all the desired quality levels and sending them to the SFU to decide what to forward to the other participants - that is the difference between a SFU and a MCU, and why SFUs can scale. SVC-capable codecs are coming to WebRTC too, which makes this a lot more efficient for the browser to do.

Encryption keys are distributed out of band of the media, so it's the same as whatever the tradeoffs are for multi-party E2E-encrypted text chat there (I don't know what those are though!)