We've had a recent simulcast regression caused by enabling transport-cc (Bug 1637636). Hopefully we can write a test that checks that all streams are being allocated bandwidth to prevent a similar failure in the future.

VP9 simulcast is broken and may have been for a long time (Bug Bug 1633876). We should probably test simulcast for all codecs, even if we expect VP9 and H.264 to fail.

Having looked into things a bit, our current mochitests really restrict stream size, presumably due to problems with intermittents in CI. What we probably want is some sort of performance test on reasonable hardware, that ensures we're sending high resolution streams as expected. Maybe we can adapt: https://fippo.github.io/simulcast-playground/firefox?