On my VDS, both 6to4 and Teredo seem to work poorly (packet loss is about 40%).
just-ping.com shows packet loss 10%-80% both for 6to4 and for Teredo address, but 0% for IPv4 address.
ping6 -n ipv6.google.com and
tcpdump -n 'host 126.96.36.199' I see more
ICMP6, echo request packets going to 188.8.131.52 than
ICMP6, echo reply coming back from 184.108.40.206.
How to debug it? Is 6to4 unreliability probably caused by misconfiguration on my side or problems on some remote side?