SAML is not the only standard for SSO. Before SAML we had Kerberos and nowadays you can use Open ID Connect. Other standards can have their own gotcha, but SAML is uniquely horrendous.
When we get vulnerabilities in the SSO protocol (SAML or otherwise) these vulnerabilities generally only affect some of the clients (identity consumers) who have implemented the protocol incorrectly or are using a feature that the provider has implemented incorrectly. Vulnerabilities that break the entire provider are less common.
When comparing this situation to having multiple different accounts, I can't see how SSO is less secure. Sure, when you have breach that affects the entire identity provider the damage is high, but the risk of having a breach (any breach!) is lower, since implementations are fewer, more consolidated and usually developed by people with better expertise.
When we get vulnerabilities in the SSO protocol (SAML or otherwise) these vulnerabilities generally only affect some of the clients (identity consumers) who have implemented the protocol incorrectly or are using a feature that the provider has implemented incorrectly. Vulnerabilities that break the entire provider are less common.
When comparing this situation to having multiple different accounts, I can't see how SSO is less secure. Sure, when you have breach that affects the entire identity provider the damage is high, but the risk of having a breach (any breach!) is lower, since implementations are fewer, more consolidated and usually developed by people with better expertise.