So I pretty strongly disagree about your statement. Repetitively sweeping an IPv6 network to DoS/DDoS the ND protocol thereby
flooding
the ND cache/LRUs could be extremely effective and if not payed serious attention will cause serious issues.
Yes.... This is an issue for point-to-point links but using a longer prefix (/126 or similar) has been suggested as a mitigation for this sort of attack.
I would assume that in the LAN scenario where you have a /64 for your internal network that you would have some sort of stateful firewall sitting infront of the network to stop any un-initiated sessions. This therefore stops any hammering of ND cache etc. The argument then is that the number of packets hitting your firewall / bandwidth starvation would be the the alternative line of attack for a DoS/DDos but that is a completely different issue.
So for /64 subnets used for point-to-points you disable ND, configure static neighbors and that's the end of it. No ND DDoS.