Yeah I agree this is a edge case, 99,99% of Caddy users don't push so much data ;)
But Caddy can't be used as a big files download server for instance, unless sticking with HTTP/1.1 (that and sendfile + kTLS not been supported last time I checked?) (at least with a single http instance).
Another issue for HTTP/3 is https://github.com/lucas-clemente/quic-go/wiki/UDP-Receive-B...
Current Linux in WSL for instance has this issue.
This will probably resolves over time as more and more Linux distros will take HTTP/3 requirements into account.