Single sign-on (SSO) is a method of access control where users are able to login once and gain access to all systems they have authorization to use without any additional login prompts.
The risk involved are
Just as it is a bad practice to use the same password on all your various web services.
It is also dangerous to let one username/password combination unlock all the resources an individual employee has access to.
If an employee forgets his password that he will not be able to reach any work resources, which can mean lost productivity until he is able to contact a Help Desk employee and regain access.