Keep in mind that most of the time, penetration happens because of operator error. Most common vulnerabilities are patchable or preventable in one way or another.

SSH contains many features which may make it vulnerable to attack. If configured incorrectly it will expose the machine, and depending on the machine this could expose a big portion of your network. I could write 5 pages on abusing SSH options for entry.

After SSH itself, there's the operating system. PAM has had holes for years and yet people everywhere rely on it for their authentication. Their OS might not be patched up, and probably has had no hardening of the kernel or userland against typical attacks.

The box is also probably on a shared network segment, meaning your attack surface is now a whole lot bigger: many machines and user sessions to hijack.

On the other hand, a VPN on a separate network segment provides protection for all these components, if you do it right. Usually the VPN should be an appliance of some kind, with service by a company who spends its time hardening the device and patching the software regularly. This makes sure the configuration of the software is correct, the system itself is resistant to attack, and authentication/authorization is managed outside the machine, which means limited access to the network for specific user sessions.

You cut the attack surface down by separating their network access away from a specific network and shared server. You also cut down on mistakes in managing said service because everything is a managed service, not a config file edited by one or more admins.

Also consider that SSH just doesn't have the features of a standard VPN that make it useful on a network level. Its tunneling feature is a joke; using ppp is the closest way to provide an actual remote vpn tunnel. Persistent connections are nonexistent. Network access control is nonexistent. Pushing routing, DNS and other network information is nonexistent. It is not meant to be a network-level tunnel, it's a single-session user tunnel for remote hosts, not networks. The extra options and features for more network access are hacks.

If you wanted to emulate a VPN using SSH, you could:

* put a machine on a separate network segment
* apply Grsec and other patches
* harden the filesystem and stop all services other than SSH
* disable user logins
* disable all features of ssh
* enable all strict ciphers, modes, options for ssh
* add ssh config options to run pppd as soon as the user logs in
* configure PAM to use RADIUS or SQL to auth with a remote database
* use iptables and some kind of packet-marking module to track network sessions by user id
* create a custom application that configures iptables rules per user
* constantly update all patches

That would get you close to a real, normal VPN. Unfortunately, it's also a metric shit-ton more code and services to do the same thing as one service, and considering you'd be setting it up for the first time, there's probably gonna be some mistakes made.

SSH is not a VPN, and a VPN is not SSH. Like I said before, if you're just remoting into your personal box, I don't think you'll have issues, because nobody would care to break in. Bigger networks are a different matter.