Doesn't fly.io have tailscale integration?
Servers sitting less than 1km to each other tend to have lesser latency between them, compared to servers that could be anywhere on the public internet.
But if you have a zero-trust architecture and everything communicates over wireguard, then technically public or private won't matter right?
Not saying public/private IPs don't matter – since almost all the servers would still only have private IPs.
In a sense this is more like VPC peering.
I remember a failure of one transatlantic line that was used for one direction of packets we were sending between EU and US. I spent a night on a phone trying to convince AWS and our other datacenter operator to work around the problem by changing their routing and to push the backbone operator to fix the problem - we didn't have any relationship with the backbone of course.
You don't want such disruption to happen in a core part of your app. These disruptions can also be intermittent and "random" and you have no way to fix.
I think I know what you're getting at, and if I'm right, this isn't really a subjective or squishy subject.