GCP for example hands out /96s to each VM, so this isn’t a theoretical or niche usecase.

Yes, but for GCP all the VMs with a /96 in the same /64 will be closely related: in the same project, same VPC network, same cloud region.

So from the point of abuse logic it's appropriate to treat the whole /64 as a single unit. (That was the starting point of the thread, even though I realize that due to thread drift that's probably not what your comment was about.)

What stuff?

Crappy network devices may not handle it properly, SLAAC obviously, I'm sure there are IPv6 tools that expect /64 for host, almost all blacklist setups for IPv6 assume /64.

This RFC has things that may not work properly not to use /64. https://datatracker.ietf.org/doc/html/rfc7421#section-4.2

The RFC only mentions issues with a shorter prefix, not a longer one. There should be no issues whatsoever with longer prefixes.