Showstopper in the sense that it may not play natively and require transcoding. While x264 has pretty much universal support, AV1 does not… at least not on some of my devices. I agree that it is a good encoder and the way forward but its not the best when using older devices. My experience has been with Chromecast with Google TV. Looks like google only added AV1 support in their newest Google TV Streamer (late 2024 device).

AV1 is definitely a showstopper a lot of the time indeed. H265 I would expect to see more on 2k or 4k content (though native support is really high anyway). My experience so far has been seeing transcoding done only becuase the resolution is unsupported when I try watching 4k videos on an older 1080p only chromecast.

I’m skeptical that you are doing much video transcoding anyway. 1080p is supported on must devices now, and h264 is best buddies with 1080p content - a codec supported even on washing machines. Audio may be transcoded more often.