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.