Comment by conradludgate

When it comes to QUIC, QUIC works best with unstable end-user internet (designed for http3 for the mobile age). Most end-user internet access is behind various layers of CGNAT. The way that NAT works is by using your port numbers to increase the address space. If you have 2^32 IPv4 addresses, you have 2^48 IPv4 address+port pairs. All these NAT middleboxes speak TCP and UDP only.

Additionally, firewalls are also designed to filter out any weird packets. If the packet doesn't look like you wanted to receive it, it's dropped. It usually does this by tracking open ports just like NAT, therefore many firewalls also don't trust custom protocols.