First, both SSH and OpenVPN are much more complicated than spiped, and thus more likely to have security flaws. OpenVPN as you know incorporates OpenSSL and makes use of an even greater fraction of its code than stunnel; Colin's primary goal in building spiped seems to have been avoiding the need for stunnel (or even stud).

Second, if all you're trying to do is hook one discrete component up to another, a full-on VPN isn't just overkill, it's also unnecessarily dangerous, since now you have to configure the network stacks to ensure that only the traffic you intended to allow is going to be able to get through after a minor compromise.