TurnKey appliances are not only great for testing and intranet deployments but also for production. In fact the first thing the appliance does is apply all the security updates to itself when you install and after that it auto-updates security fixes daily.
Remember, TurnKey is just a customized distribution of Ubuntu under the hood (soon Debian also). You wouldn't get higher security if you took stock Ubuntu and customized it yourself. Probably the opposite.
Though there are no known vulnerabilities in any of the admin interfaces (e.g., webmin, ssh, webshell, phpmyadmin, etc.), for production use we do recommend turning off or firewalling any service you don't use. It's good practice even though it isn't strictly necessary.
Designed for production use
Remember, TurnKey is just a customized distribution of Ubuntu under the hood (soon Debian also). You wouldn't get higher security if you took stock Ubuntu and customized it yourself. Probably the opposite.
Though there are no known vulnerabilities in any of the admin interfaces (e.g., webmin, ssh, webshell, phpmyadmin, etc.), for production use we do recommend turning off or firewalling any service you don't use. It's good practice even though it isn't strictly necessary.