Could you share a list of IPv6 issues that IPv4 does not exhibit? Something that becomes materially harder with IPv6? E.g., "IPv6 addresses are long and unwieldy, hard to write down or remember". What else?

Traffic shapping in v6 is harder than v4. At least it was for me, because NDP messages were going into the shaping queue, but then getting lost since the queue only had a 128 bit address field, and 128 bits isn't actually enough for local addresses. When the traffic shaping allowed traffic immediately, the NDP traffic would be sent, but if it needed to be queued, the adapter index would get lost (or something) and the packets disappeared. So I'd get little bursts of v6 until NDP entries timed out and small queues meant a long time before it would work again.

Not an issue in ipv4 because ARP isn't IPv4 so IP traffic shaping ignores it automatically.