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.
Well, T-Mobile US is 100% IPv6:
* https://www.youtube.com/watch?v=QGbxCKAqNUE
Facebook is IPv6-only on their internal infrastructure:
* https://www.internetsociety.org/resources/deploy360/2014/cas...
Microsoft has been moving to IPv6-only for their corporate network (so IPv4 address can be used for revenue-producing Azure):
* https://www.arin.net/blog/2019/04/03/microsoft-works-toward-...
So he better tell those folks that IPv6 is not a thing.