The only pain I've ever seen is in corporate networks where all the tooling around the network management are IPv4 only but those would break even if you add a single bit to an IPv4 address.
Software isn't ready or was written back in the early days before they figured out how IPv6 would actually be deployed. I had an Asus router back in the days, it had primitive and unstable IPv6 support, which just grabbed a single /64 and that was it.
I then ran pfSense for some years and it was unusable for IPv6 due to everything being geared around fixed prefix. Even if my ISP had given me a fixed prefix it would have been a pain, because I got two new cable modems in that period (one due to the previous being too old, other one due to a move), so would have gotten a new prefixes anyway and would have had to rewrite all my firewall rules. Would have been major PITA each time. No such need with IPv4.
I switched to OpenWRT some years ago and it's mostly worked since. Mostly.
Android phones in the homes don't respect my DHCPv6 settings, and it took me some time to figure out it ignored my DNS settings because I had only IPv4 configured as my local DNS server. Other boxes were fine with that, but not Android, which silently used Googles DNS instead.
Running dual stack isn't ideal either, since it can lead to inconsistencies. When some things work and some things don't, and it can be difficult to figure out why because of the non-trivial interaction between IPv4 and IPv6.
It's also painful because just about everything is different. So very little of what I know of how to configure my IPv4 network carries over. It's very much not just IPv4 with more bits. I'm old enough now that I'm not terribly excited about digging around in the IPv6 technical weeds, and I haven't found a good guide or reference I can fall back on.
Mostly because of the number of machines that I have to fix up and the fact that updating each of them involves a fair bit of time. My estimation is I'm looking at at least a week's worth of work, during which my network isn't fully functioning.
It's also complicated by the number of devices I have that aren't possible to make work with IPv6 at all, which means I have to maintain some IPv4 segment and deal with making the two work together in a seamless way.
I'm also very concerned about security. I'm not confident that I know enough about how to secure an IPv6 network adequately, so I need to set aside a fair bit of time for study before I even start.
All in all, it's a large project with a lot of friction that wouldn't be as large if I didn't have to rethink everything.
But the last time I mentioned half-measures here on HN, I got dumped on pretty harshly.
