On the cheapest budget cobbled together network, either one turns into a network broadcast. But even a bargain basement 1990s Ethernet chipset does onboard multicast filtering, so irrelevant neighbour discovery packets needn't wake up your OS whereas every ARP query must be drawn to the attention of every host's OS to confirm it's not for them.
However, spend a little more money and the network switch understands multicast, so now the data isn't even sent across links which don't have anybody listening to that address, whereas of course nothing can be done about broadcasts.
You couldn't have easily done this trick in IPv4 because the address space is too small, but in IPv6 choosing the Ethernet multicast addresses to make this work falls out very easily.
It still would have taken a long time to deploy, because you still need hardware and software support in lots of places, but it might have been a little less long, and a little less bad along the way.
I don't think we got much of use from the new ways either. I can announce IPv6 ranges to my LAN from two different ISPs, but my clients will use an IP from one and send to the gateway from the other, so that doesn't help me.