Unlike classical key distribution, these guarantees derive from fundamental physics laws as opposed to, say, RSA that can be broken given enough computing power/time, and so are unbreakable.
Unfortunately there can be flaws in actual implementations of the BB84 scheme, such as side-channel attacks. E91 [1] (a newer scheme) addresses these flaws and prevents these attacks.
[1] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.67....
