I wouldn’t call it a clone, Tailscale didn’t invent mesh VPN’s. I believe Nebula is fully self hosted, while Tailscale makes initial connections through their servers. That means Nebula is more secure and private if you’re paranoid, but also harder to set up. They’re also based on different VPN protocols.
Tailscale actually published a surprisingly unbiased comparison: https://tailscale.com/compare/nebula
Lol. It should also be no issue for you to find the comment and read their answer