When it comes to video it’s not only the browser. It’s also your gpu, your OS and your gpu drivers.
Notably, YouTube these days prioritize AV1 codec even if you don’t have gpu acceleration for it, making lots of systems fall back to CPU decoding and making it completely unusable. Install the h264ify extension to force h264 during content negotiation and get your gpu decoding back.
Even if you can make a matrix of all those combinations, it’s even more complex than that to test in practice. Take my laptop for example, it starts off good and manages the cpu decoding for a while, a few minutes into a video it overheats and throttles, causing stutter.
What YouTube should do on the other hand, and I’m sure they already do, is to collect metrics from all playbacks. That should show black on white how many users struggle with each codec.
I don’t think I’m in any minority here given how many million installations the h264ify extension has. Google simply care more about their bandwidth cost than the user experience.