My experience is with a large dual-stack network that has several
class B IPv4 networks.  Router advertisements seem to be slow and
unreliable, for one thing.  Old protocols not supporting v6 are also
an issue.  Then there was the time when a user could mis-configure
their Windows computer to turn it into a router that led to a dead end.
Random network freezes were the result.  What's to prevent a bad actor
who managed to gain physical access from installing her own router
and then siphon off traffic for their own ill deeds?

* avoid problems when the ipv4 pool is exhausted.

But that's why God invented NAT!