Someone should double check me, but I think PD less than a /64 also just breaks (and probably is against the spec).
A lot of the complaints I have seen in the last decade is from ISPs doing silly things and cutting their teeth on fresh IPv6 deployments. My ISP seems to have their collective ducks in a row now, and it has been rock solid for years.
I actually had a case recently where a misbehaving IPv4 IoT device consumed my entire DHCP pool. IPv6 devices kept chugging along without any problem.