OpenVPN remote access requires that the server have a publicly routable IP address that allows new connections. For many/most people, that’s not a problem.
For some ppl, it’s a problem - for example, the story above about subleasing internet access.
I’m considering zerotier for a similar corner case. I’ve got cable modem as my main internet, but I’d like to set up failover to a cellular modem. I can confirm that the cellular modem works for outbound internet access. The cellular IPV4 address is behind carrier grade NAT, so an openvpn server won’t work. Although I get a routable IPV6 address, all connections to it fail. I think zerotier UDP punching may be the best way to remotely access my network over the backup cellular connection.