What are the drawbacks of isolated identities from the user's point of view?

The user gets a different digital identity, and often a different authentication means, for every relying party, and has to fully trust each provider's handling of their identity data with little transparency.

Two consequences:

• Identity sprawl: one account per shop, forum and tool means dozens of usernames and passwords, which pushes users towards password reuse, the root of credential-stuffing attacks.
• Trust without insight: the user must trust that each provider stores the data securely and uses it only as promised. Modern data-protection laws demand transparency, but from the user's chair it remains hard to verify.

This is exactly the pain that "log in with Google" promises to remove, and the trade-off that comes with it (a central party seeing every login) is why the choice of topology matters.

Go deeper:

• Credential stuffing (Wikipedia) — what password reuse across isolated accounts leads to.