I had the same reaction, all the other parts of the parent comment sound bad but switching to mDNS seems like the one that should have been an improvement or at least neutral...
The literal Chromecast itself seems to be exceptional device that just works.
UPnP generally or just UPnP IGD (frequently referred to as “UPnP” in consumer router UIs)? I’d imagine the primary reasons a smart speaker would want to use UPnP are largely unrelated to punching holes in firewalls and NATs (what IGD is about). And however distasteful RPC over XML over jury-rigged HTTP over IP multicast may be, it’s hardly inevitable that it must create a security problem.
Even if we’re speaking about holes, though, I feel like I must object to the broad description of that function as a security vulnerability, as any instance of that moves us that much farther away from a true peer-to-peer Internet.