2 is a security nightmare but that’s why firewalls prevent it by default
3 well you can set the dont fragment bit at a client side or a router can drop the packet. These are choices. If a 1500 byte IPv6 packet arrives on a router with an 1100 byte next hop, does it just drop? Or send back a fragmentation needed icmp? How is that different from setting a “don’t fragment” option on a router.
4 isn’t created from a security or management point of view either. And v4 has the 169.254 range for this purpose. I guess the lack of router advertisement is the primary difference. And the operational expectations.
5a I’m not sure about. My main experience with multicast is pim-sm on v4. SSM v4 multicast however seems simple, and while I don’t use it as I have kit that’s too old for it is v6 really easier than v4/ssm/igmp3?
As for arp, I don’t see any real complexity with it as a network operator, but maybe that’s because I’m used to it. Perhaps it’s easier to implement nd rather than arp, but given almost every v6 deployment for the last 30 years is dual stack all it does is increase complexity.
> If a 1500 byte IPv6 packet arrives on a router with an 1100 byte next hop, does it just drop? Or send back a fragmentation needed icmp?
Yup [0].
> How is that different from setting a “don’t fragment” option on a router.
It's the exact same, of course with the difference that it's the default and that nothing needs to support packets with the “don’t fragment” option disabled (since it's mandatory).
> And v4 has the 169.254 range for this purpose.
Sure, but seeing 169.254.x.x usually means that something is broken, while seeing IPv6 link-local address is perfectly normal.
> As for arp, I don’t see any real complexity with it as a network operator, but maybe that’s because I’m used to it.
Well it's part of the reason why 802.11 tries so hard to pretend that it's Ethernet, and I've seen ARP storms a few times but never any NDP storms.
> but given almost every v6 deployment for the last 30 years is dual stack all it does is increase complexity.
Yeah, IPv6 is great, but dual-stack is fairly annoying, and given that IPv4 is the older protocol and still essentially mandatory, I definitely get why people dislike IPv6 (even when it's really IPv4 that's the problem).
[0]: https://en.wikipedia.org/wiki/Path_MTU_Discovery