@lxo
The only major problem is that a third-party server is always a single point of failure, no matter how nicely it may present itself. If some authority requests some sensitive data from the server owner you are essentially defenseless unless the server owner is that principled to resist gag orders. Affinity groups on the other hand do not require you to even know each other. In decentralized self-hosted networks affinity groups are a circle of friends you hang out with. You do not need to trust each other, you give them only the data you want to give them, nothing more. Unlike with federated servers.
@jmangt