Sure, how about routes are not working or are very very slow via IPv6? Just an example I had a problem with recently: DTAG (largest german carrier) had no route at all to a DC in the Netherlands via IPv6. I had no option to reach DTAG from this specific DC, which is basically most germans. It took days (!) to get sorted.

Next example is Cogent, they had huge packet loss via IPv6 for days (we are speaking 50%+). After a ticket was opened it was fixed very fast, but it seems no one cares about IPv6 as long as they can put a stamp on it which says "We support IPv6". Most people who shout for IPv6 are not even using it so I am always a bit sceptical.