Not much, really. If you're considering alternatives, check out SoftEther VPN [1] also.

https://www.softether.org/

Open source, true L2 emulation, more platforms, faster, less bloated, rules engine, ...

Plug: If adhoc peer to peer port forwarding is what required, try https://github.com/selvakn/p2p-port-forward (which uses zerotier-sdk)

Instead of being a VPN it is a virtual network device. It creates a private virtual ipv6 network. It acts like being on a local network, so all devices inside that network are visible. Software defined networking and all that. Quite impressive.

That's what Hamachi does. They're still called VPNs.

You have no idea unless you try it. It is different. for example this network device is meant to make a network between all your iot devices, or over a cross provider cloud infrastructure. It does not route all tradfic through a vpn server. Nodes connect directly to each other. you do have a fallback openvpn mode for iphone devices though. You cannot really imagine if you havent tried it (and that is a problem for zerotier anyways)

Have you tried Hamachi? That's exactly how it works. It's not a VPN that routes all your traffic, it's a virtual LAN. You get a private IP, and can communicate with other devices on the same network over their private IPs. And yes, nodes connect directly to each other.

Open source maybe?