Udp - HTTP/3 Explained
There are enterprises and other network setups that block UDP traffic on other ports than 53 (used for DNS). Others throttle such data in ways that makes QUIC perform worse than TCP based protocols. There is no end to what some operators may do.
For the foreseeable future, all use of QUIC-based transports will probably have to be able to gracefully fall-back to another (TCP-based) alternative. Google engineers have previously mentioned measured failure rates in the low single-digit percentages.