SSH does not allow to open a shell on the client from the remote server. It does support reversed port forwarding, but that is initiated on the client side via -R or a ~-command.
The main risk is X11 forwarding. If your SSH client is configured to allow programs on the server to render gui windows on your screen, there is an issue. Even untrusted X11 programs can cause a lot of damage. So it is best to use -x (important: small x) on the command line or
ForwardX11 no in the ssh_config file.
-X Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring.
For this reason, X11 forwarding is subjected to X11 SECURITY extension restrictions by default.
-x Disables X11 forwarding.
-Y Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.