Let me try providing a different perspective based on experience. QUIC works amazingly well for _some_ kinds of machine to machine traffic.
ssh3, based on QUIC is quicker at dropping into a shell compared to ssh. The latency difference was clearly visible.
QUIC with the unreliable dgram extension is also a great way to implement port forwarding over ssh. Tunneling one reliable transport over another hides the packer losses in the upper layer.
QUIC v1 does encrypt the SNI in the client hello, but the keys are derived from a predefined salt and the destination connection id. I don't see why decrypting this would be difficult for a nginx plugin.