A service I am aware of has HTTP streams. Video on Demand streams are rate limited by the server to a fixed rate, which is sometimes significantly under the peak bitrate of the file; and the recommended player device doesn't have a large enough buffer for that.

That's 100% a service provider issue.

Live streams have some issues too, but I haven't debugged those. On the other hand, ESPN+ live streams aren't exactly five nines of quality either.

The pirate IPTV streams are not UDP, they’re HLS vis HTTPS. Often behind Cloudflare.

Not on my side. We dont throttle anything.

How is that going to work with QUIC?