Wireguard is great for a point-to-point or multipoint VPN set up by a competent network administrator between machines with static addresses. But that's the only thing it does. It does not handle authentication or mobility very well.

No, it’s not. It handles the keys, provisioning, DNS, NAT traversal, and a bunch of other stuff. WireGuard is a great technology - Tailscale is like a usability layer on top of it.

I pay Tailscale nothing, why switch?

I think an important feature for homelabs is NAT traversal