They claimed and showed QUIC slightly-to-moderately reduced latency, particularly for mobile. This benefits Google by loading pages with third-party content, i.e. ads, faster.

But QUIC significantly increases CPU utilization on servers, at least the widely used userland stacks do. Unless/until Google deploys QUIC in the kernel (or puts the whole network stack in userland, a la DPDK), this won't change.

The multicast claim is kinda bizarre. I can see how QUIC could help eliminate UDP client barriers, but those barriers pale in comparison to multicast. Multicast routing just doesn't exist on the Internet; it's only supported within some independent, typically small networks. Most ISPs don't support it. Wherever you could manage to distribute content with multicast, you'd necessarily also be resolving the collateral routing problems which QUIC support resolves, whereas even ubiquitous QUIC doesn't materially improve the multicast situation.