Codec support is a very annoying issue on mobile. Hardware support for VP9 (let alone AV1) codecs is relatively recent on Android devices, and Apple doesn't provide HW acceleration for Google's codecs at all (or at least not when I was trying to do it).
If we want to get really pedantic...WebRTC will support SW H264 if you enable it explicitly in the compilation, since you're technically on the hook for royalties if you compile it yourself. Footguns galore :)