> It’s also similar to running your own derp server (which works also in TCP), but without the hassle of doing so, and perhaps without having to open ports to the internet (needed in derp) so long as the relay is reachable by peers.

I think most folks will need to open a port to the internet, because otherwise you wouldn't need the tailscale to begin with. e.g. connecting your cloud network to your on premise network etc.

Of course exceptions apply, like both clients can reach the peer relay, but not each other directly.