not true, currently what I see is that the users with ipv6 are less abusive than those from ipv4 (it could be that there is still not malny people using ipv6)
As for long vhosts you can do exactly same with ipv4.
And is pretty easy to block long vhosts for ircadmins.
Usually you can easilly check how big ipv6 class user owns, and what is her real ipv4 address (it's for 3ffe:: amd 2002:: addresses).
Since many ircops k-line entire class for abuse, I'm sure that ipv6 is less abused.