The primary disadvantage of Single Sign-On (SSO) access control is:

Single Sign-On (SSO) is a convenient and efficient way of authenticating users across multiple applications and platforms with a single set of credentials. However, it also poses some security risks and challenges that need to be considered and addressed. One of the main disadvantages of SSO is that it creates a single point of failure and a high-value target for attackers. If an end-user credential is compromised, the attacker can gain access to all the systems and resources that the user is authorized to access, potentially causing significant damage and data breaches. Therefore, SSO requires strong security measures to protect the user credentials, such as encryption, multifactor authentication, password policies, and monitoring. Additionally, SSO users need to be aware of the risks and follow best practices to safeguard their credentials, such as using strong and unique passwords, changing them regularly, and avoiding phishing and social engineering attacks. References:

1: What are the disadvantages of single sign-on authentication? - Information Security Stack Exchange

2: Single Sign-On Disadvantages: 6 Advantages and Disadvantages [What You Need to Know] - Mostly Blogging

3: SSO Security Risks: The Drawbacks of SSO (And What Can You Do About it) - Zluri