There is an overview in the whitepaper:
https://github.com/yggdrasil-network/yggdrasil-go/blob/maste...It fits the "private VPN" use-case quite well in my experience. You can connect to the wider network over the Internet, or just set your nodes up. If yggdrasil is installed on every router, it automatically creates a nice network topology, since it finds peers on the local subnet. Router advertising is also a possibility.
Though there's no real drawback to connecting to the wider network since it's end-to-end encrypted, you have to be aware that specifying more than one peer will make it possible for traffic to be routed trough you, so the whole network performance can be sensitive to the choices that are made when peering over the Internet, as I think hop count is the only metric for now.
For private meshes, I don't think you can specify fallback peer addresses over the Internet, so you have a bit of the same risk here. I've seen some info on mesh wireguard networks with peer information stored in DNS at this year's FOSDEM, but that's currently definitely more configuration than yggdrasil.
End-to-end encryption and the ability to generate your own static, roaming-compatible IPs is nice. I just wish one could open sockets directly with a crypto key rather than the derivated IP.
For more discussion, I can really recommend the Matrix chat room :)