In general, anonymity systems have huge practical constraints and even
things like TOR are subject to various traffic analysis attacks.
Privacy is really hard and the best we can do is still vulnerable to
some quite practical approaches.
TOR for example is reasonably vulnerable to attacks to figure out origin
near the origin and to figure out the server near the server.
It's reasonably vulnerable to attacks to figure out both if you control
enough nodes in the network.
Where enough isn't that large if you have some money to throw around.
To be clear, the TOR Project is very good at trying to accurately
describe what privacy they can and cannot provide.