Personal experience that I see noone talk about with IPv6 is how much more expensive hardware that handles it correctly is for datacenters. On IPv4 your usual unit of allocation is a /32 for customers - that means a simple hashmap `1.1.1.1=destination mac` works wonderfully and is cheap (single memory lookup), but for ipv6 your usual unit is a /64 so its longest prefix match instead, which requires parsing the address to group it back into the /64 and alot of switches and routers that are already expensive still have very low limits on LPM memory banks.

Expensive switch at work we have can only do 3000 route entries for example on ipv6. If we did /128s it's basically infinite though, because it goes from a LPM to exact match, which has much much more memory available.

Doesn't help as well that for example, to be able to do SLAAC or even DHCPv6 (which barely works reliably in my experience) you need to do a /64 at minimum, those mechanisms dont even work otherwise, so for ISPs who can easily have more than 3000 downstream customers doing routed ipv6 is such an increase in hardware cost vs just doing NAT which they were already doing anyway.