[fsflover]

Your own Matrix server does not spread any information unless you allow it or unless you message other servers.

Now, concerning the other servers, it may be a problem, just like Gmail is a problem for everyone running their own email server. However, it's a much smaller problem and at least theoretically everyone who cares can escape the walled garden.

You can create a server for all your friends and you will always know exactly which information is shared with others and which isn't.