Nebula transits every EC2-to-EC2 packet at Slack, across lots of AWS regions and tens of thousands of hosts. It’s probably doing petabits of traffic per second. And it’s a safer, more expressive firewall than EC2 security groups.
So, yes, it works for personal use-cases but it works for truly gigantic applications, too.
The big thing I see missing is the management piece, which is what makes Tailscale compelling. If you're just running Nebula as an individual user, or for a small org, there wouldn't be much overhead. Otherwise, for larger deployments, you need to "roll your own" solution to manage configs outside of Nebula itself.
You'd also want this to be self-service in some way - so road warriors can rotate their own certs, with auth backed by some kind of central SSO system. The last I looked, Nebula didn't offer this stuff.
So, yes, it works for personal use-cases but it works for truly gigantic applications, too.