There are a number of alternatives that I was playing with so far that more or less aim to provide the same, just listing them here in no particular order that happens to be alphabetic:
I used PeerVPN before for server-to-server connectivity in the data center and worked fine, but switched to tinc because it is packaged in all distributions and seems to be more actively maintained, i.e. has occasional releases. It took a bit more time to tame it as it uses public key crypto instead of a PSK.
ZeroTier is slightly different, it provides the same functionality, but requires a network of “root” servers to help with establishing connections between clients and the focus is more on managed networks through their (proprietary) management UI and network controller (free software). They do have Android/iOS apps available, but those are not free software either last time I checked.
meshbird seemed promising, but was just a bit too fiddly to compile and seems like development has stalled now.
Very cool. It would be interesting to see the app coin idea attached to something like this to incentivize creating the network.