Suddenly, your smart lightbulb is accessible by everyone. Not a great idea.
> With IPv6, if you want a peer-to-peer connection between firewalled peers, you do a quick UDP hole punch and you're done - since everything has a unique IP, you don't even need to worry about remapping port numbers.
There is no guarantee with IPv6 that hole punching works. It _usually_ does like with IPv4.
The answer here is kinda that Wi-Fi isn't an appropriate networking protocol for lightbulbs (or most other devices that aren't high-bandwidth) in the first place.
Smart devices that aren't high bandwidth (i.e. basically anything other than cameras) and that don't need to be internet accessible outside of a smart home controller should be using one of Z-Wave/Zigbee/Thread/LoRaWAN depending on requirements, but basically never Wi-Fi.
> Suddenly, your smart lightbulb is accessible by everyone. Not a great idea.
Why would it be "accessible by everyone"? My last ISP had IPv6 and my Asus (with the vendor firmware) didn't allow it. My printer automatically picked up an IPV6 address via SLACC and it was not "accessible by everyone" (I tried connecting to it externally).
A firewall solves that issue, IPv4 or IPv6.
I finally have IPv6 at home but I am being very cautious about enabling it because I don't really know what the implications are, and I do not trust the defaults.