Who's going to configure this properly? Some apps will need to punch holes in the firewall.. It simply hasn't been tested enough. Especially in home / small office situations.
What even is there to configure? An IPv6-capable SOHO router ships with a “forward only related, established” config by default, to put it in IPTables terms. Most of the time, you won’t even have the option to disable this entirely.
There is nothing to test either because this is exactly how SOHO routers have been working for decades.
If suddenly everybody has an ipv6 accessible device, there will be sooo many hacks