[grumblez]

"lesser known" as in protocols such as IPSec, ZeroTier, WireGuard, etc. Of which IPSec has been around forever and many NATs/Firewalls have special handling rules built in, just as @api mentioned in another comment. Yes, ZeroTier uses UDP underneath, but that doesn't mean symmetric NATs don't/won't cause havoc to peer to peer protocols using UDP.

[kortilla]

Wrong layers of the network. IPSec is comparable to TCP/UDP, not wireguard/zerotier. It’s L4 and NAT can’t have enough intelligence to setup IPSec meshes without explicit configuration.

Finally, how can ZeroTier’s support be so incompetent to not recognize connectivity issues between endpoints? That’s one of the few things that goes wrong with tunnel meshes.