It is always the case that an app can break due to something in the app itself. An app may for example link with a different TLS implementation that breaks. Or

It is always the case that an app can break due to something in the app itself. An app may for example link with a different TLS implementation that breaks. Or an app may use the local DNS resolvers but do DNSSEC local validation. With QUIC (HTTP3) a large part of the network stack will be part of the application. So you lose that visibility as well.