De privacytools.io :

• Adding new features is more complex, because these features need to be standardized and tested to ensure they work with all servers on the network.
• Some metadata may be available (e.g., information like "who is talking to whom," but not actual message content if E2EE is used).
• Federated servers generally require trusting your server's administrator. They may be a hobbyist or otherwise not a "security professional," and may not serve standard documents like a privacy policy or terms of service detailing how your data is utilized.
• Server administrators sometimes choose to block other servers, which are a source of unmoderated abuse or break general rules of accepted behavior. This will hinder your ability to communicate with users on those servers.