Traditionally two of the issues with federation have been sharing namespaces and sharing logins. Once you need a single source of truth, the federation breaks and you need to rely on a central authority. There are some new decentralized protocols (Handshake's decentralized naming system is one) that provide a solution to these problems so I'm bullish on federation in general — I think a federated system could break into the mainstream given the recent geopolitical issues affecting centralized services.