If the the customer's server is security critical do not setup TeamView to connect to it.

A cursory reading shows that TeamView can use UPnP (Universal Plug and Play).

There are **29 CVE**s (Common Vulnerabilities and Exposures listed for uPnP!

If you do not know exactly how the authentication and routing of a product works, do not use it for security critical activities or infrastructure.

Anything that is more complex has the risk of having more bugs, issues, and vulnerabilities. So when you mentioned that ssh was simpler you had the right idea. 

If you are using ssh to access security critical infrastructure please use key authentication instead of password authentication.