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.
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