QUIC/HTTP3 relies on TLS. If you already have some encrypted transport, like an Istio/Envoy service mesh with mutual TLS, or Zerotier/Tailscale/Wireguard style encrypted overlay network, then there are no benefits to using HTTP3. Moreover native crypto libraries tend do a better job handling encryption anyway so rather than wasting cycles doing crypto in Go or Node it makes more sense to let the service mesh or the overlay handle encryption and let your app just respond to clear requests.

Sure I was responding to the context as I understood it here which was listening on HTTP/3 as an application rather than a service mesh layer. HTTP/3 can definitely be a choice for service mesh or some sort of overlay. Personally if I were setting up a new cloud/DC today I'd probably just use ZeroTier (or Tailscale) and let the overlay deal with encryption while I just have my sources and destinations do IP based filtering.