I tried using this or a similar repo to set up a Tailscale exit node on Fly.io before.
The downside is that my traffic never went direct; it was always relayed via a Tailscale DERP node, as Fly.io machines were only accessible via anycast, and so a direct connection from Tailscale on my machine to the exit node on Fly.io couldn't be established.
So performance wasn't as great (and I felt bad about using up Tailscale's DERP bandwidth, as a free user).
The downside is that my traffic never went direct; it was always relayed via a Tailscale DERP node, as Fly.io machines were only accessible via anycast, and so a direct connection from Tailscale on my machine to the exit node on Fly.io couldn't be established.
So performance wasn't as great (and I felt bad about using up Tailscale's DERP bandwidth, as a free user).