I just set this up the other day, and I got my ping to drop from 16 to 10ms, and my bandwidth tripled, when connecting from a remote natted site to a matter desktop my house. Together with Moonlight/Sunshine I can now play Windows games on my Linux desktop from my MacBook, with 50mbps/10ms streaming. So far so good!

Not a single port forwarded, I just set my router up as peer node.

Neat use case. But in fairness, you've simply 'offloaded' NAT traversal/port forwarding to automagic helper protocols over which you have no control even if you wanted it.

I'm confused. I wanted to do this too with an OpenWRT router, but I was under the impression I still had to open a 40000 port so my NAT devices can see it. Wouldn't it still be on the exposed public Internet?

May want to give Apollo a try: https://github.com/ClassicOldSong/Apollo (re Sunshine)

That seems really exciting! If you wanted to share game streaming to a general public would they have to install tailscale on their device/login? How does that work? Am I right in assuming that tailscale is built mostly for sharing resources with people you trust instead of the general public?

What hardware do you use on the networking side?

There are several ports open (you dont open them, Tailscale does), including for peer relay. Some are vpn ports, but the ports for relay servers are not for VPN so my guess is that the software that listens to those ports is a lot less secure (compared to Wireguard or OpenVPN).