I remember reading about the downside with HTTP/3, which was something about it making the web more centralized and reliant on the big tech. I just don't remember exactly what the user pointed out.
Probably about the protocol being more complex with less implementations or the forced encryption which means acquiring certificates for TLS. Spoofed IPs for DDoS is a bit more troublesome for UDP protocols, though smaller players will get smacked down by any DDoS anyway. I hope to see more applications and games wrap their traffic in QUIC/HTTP3 to "defeat" firewall filtering.