They are not directly comparable. Nebula is a mesh VPN. Wireguard can be used as a building block for a mesh VPN (as it is in Tailscale), but it does not have that function organically.

I have tried Tailscale / Headscale and did not find the overhead worth it. Both can saturate a 10Gbps link and that's all I need right now. Nebula's much simpler to administer. The configuration's spelled out in the client configs and in the certificates you provision. If you're already using some form of configuration management, it's quite easy to make changes. If you require a Web UI, Tailscale / Zerotier / etc may be better. There is a company that provides a Nebula-based service with a Web UI but I haven't tried it.

They're solutions to slightly different problems.

I'd characterize it as such: Wireguard is more of a plumbing solution that gives you a point-to-point connection, either it's enough for you or you can use this to build what you need.

Nebula is more of an end-user ready-to-use solution, gives you features like NAT traversal, automatic selection of direction connections if possible, the nodes don't need to know other nodes' addresses ahead of time(x) etc.

(x) except for the lighthouse

Nebula does use WireGuard, as per their go.mod file.

EDIT: actually, they just use various I/O abstractions that WireGuard developed.