I left my job as a NE/architect over a 15 years ago, but the show stopper back then revolved around how to handle routing with firewalling. Firewalling being biggest roadblock due to needing traffic symmetry. I'm doing my best to remember why we stopped at just providing v6 at the edge for site-specific Internet hosted services and never pushed it further.
Mind you, our team discussed this numerous times over a few years and never came up with a solution that didn't look like it would require us to completely fork-lift what we were doing. The whole team was FOR getting us to v6, so there was no dogmatic opposition.
Consider this:
25k employee company. Four main datacenter hubs spread out across the USA with 200 remote offices evenly dual-homed into any two of the four.
All four of the DCs had multi-ISP Internet access advertising their separate v4 blocks and hosting Internet services. The default-route was redistributed into the IGP from only two locations, site A and B. e.g. two of the four DCs were egress for Internet traffic from the population of users and all non-internet-facing servers. IGP metrics were gently massaged as to fairly equally use of both sites.
All outbound traffic flowed naturally out of the eastern or western sites based on IGP metrics. This afforded us a tertiary failover for outbound traffic in the event that both of the Internet links into one of the two egress sites was down. e.g., if both of site A's links (say, level-3 and att) were down, the route through site A was lost, and all the egress traffic was then routed out site B (and vice-versa). This worked well with ipv4 because we used NAT to masquerade all the internal v4 space as site X's public egress block. Therefore all the return traffic was routed appropriately.
BGP advertisements were either as-path prepended or supernetted (don't remember which) such that if site A went down, site B, C, or D would get its traffic, and tunnel it via GRE to the appropriate DC hub's external segment.
The difficulty was that traffic absolutely had to flow symmetrically because of the firewalls in place, and easily could for v4 because NAT was happening at every edge.
With v6 it just didn't seem like there was any way to achieve the same routing architecture / flexibility, particularly with multi-homing into geographically disparate sites.
I'm not sure anymore where we landed, but I remember it being effectively insurmountable. I don't think it was difficult for Internet-hosted services, but the effort seemed absolutely not worth it for everything on the inside of the network.