What are the advantages and disadvantages of a broker-based (hub-and-spoke) model?

Advantages: one trust relationship per participant instead of a mesh, central services such as discovery, identity linking and protocol translation, and easy onboarding of new IdPs and RPs. Disadvantages: the broker is a single point of failure and a single point of observation, it must be trusted by everyone, and its authentication proxy makes end-to-end encryption between IdP and RP difficult.

Advantages:

• Scalability of trust: adding a participant means one new agreement with the broker, not one with every existing member.
• Shared services: metadata management, IdP discovery, identity linking, transformation of assertions, protocol transformation, all built once.
• Heterogeneity absorbed: IdPs and RPs can use different protocols and identity formats; the broker translates.

Disadvantages:

• Availability: if the broker is down, nobody in the federation can log in anywhere.
• Privacy and security: the broker sees every authentication and the plaintext of every assertion it re-issues, so it must be exceptionally well protected and governed.
• Trust concentration: all parties must trust the broker's operator, which raises governance questions about who runs it and under which rules.