IPv6 -together with WireGuard- gives privacy, security, and performance. The downside is the complexity to set it up.
Tailscale builds on the shoulder of giants. IPv4, WireGuard, Samy Kamkar NAT punching, OpenSSH, and probably many more. One of the upsides is the combination of these, and that the management interface in general is easy. But what counts for CA is also true for Tailscale: both are using FOSS to in the end deliver a (proprietary) service.
But because almost everything is build on top of FOSS and there's Headscale (and they're cool with it), this isn't a major issue to me. Like, it is a downside, but not a major one, as vendor lock-in is practically non-existent. In fact, it is likely an upside from a business/support PoV.
Anyway, remember IPv4 classes? Then they made it classless. IPv6 is not 128bit, its just 64bit with 64bit host address. So, first mistake. IPsec mandatory? pure stupidity. Crypto moves fast, every 10 years many protocols are obsoleted. How you will provide E2E connectivity with that?
In 1997 IPv6 was seriously immature yet to start migration. Additionaly, it was very different from IPv4 so was mostly ignored. What IPng team should do, is just take IPv4, extended it to 64bit, call it IPv6 and we are done. As bonus, they should think about some basic IPv6 -> IPv4 interop so clients would NOT need to be dual stack. And that could work back then. Now we are fucked.