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IPv6 may be regarded as less secure than IPv4 because it does away with the need for NAT and emphasizes end-to-end network design (like the original internet). So if you don't have an IPv6 firewall blocking unsolicited incoming connections potentially every device on your LAN could be accessible from the internet and would need their own individual firewalls or access restrictions (something that many IoT devices do not have).
IMHO IPv6 was created by people with an astonishingly naive view of the internet. An internet of happy unicorns, where every device on the planet is directly accessible (and uniquely identifiable) from everywhere and everyone is nice to each other and there are no bad people. Obviously the real world is not like this and there then followed a whole bunch of RFCs and amendments to try and stick a privacy/security band-aid on the protocol.
Other notes:
- IPv6 address contains fingerprinting/identifiable information
- IPv6 has multiple different implementations
- IPv6 supports encryption, but it wasn't made mandatory
I consider IPv6 to be over-engineered. They set out to resolve one specific issue (the limited address space of a 32-bit IPv4), and started bolting so many things on top of it, provide so many variants in implementation, even ultimately adding NAT6 support for some people who complained they still wanted to use NAT, that ultimately it's become a huge mess, slowing down its implementation.
Additionally, In order for IPv6 to be implemented properly, it requires ICMP access to the device. Full compliance requires a reverse hostname. The former creates exposure that doesn't exist with IPv4, the latter weakens opsec.
Performance-wise, where do you want to begin?
6to4 tunnels are measurably slower, and that's honestly the main problem. Many ISPs are still using this method.
Client implementations (like Windows 10/11) perform excessive AAAA DNS lookups, causing it to be slower. I've noticed this on the Pi-hole -- even with a dutifully implemented IPv6 home network (dual stack, of course), network queries dropped more than 50% when I went IPv4.
And in "perfect" implementations (which are apparently hard to come by), IPv6 is slower at worst - same as IPv4 at best.
