The main disadvantage is there is no encryption or security. Any time you type usernames and passwords it's sent in clear text over the wire. Unless you are using incredibly old devices I wouldn't be using telnet its a risk.
SSH is the preferred method. Servers, PCs, and current switches/routers have SSH capability's you have to configure or install them but its worth it. SSH use certificates to encrypt data similar to web sites. If your comfortable with using certs then SSH should be quite easy for you.
The upside to telnet is it's so easy to use and nothing has to be configured at all really, but that comes at such a cost.