Agree I think it's partly both.
I imagine other breeds of dogs could be dangerous or aggressive if mistreated, neglected (by this I mean not given adequate exercise, stimulation, suitable environment etc rather than starving) abused by their owners, or even trained to be defensive/aggressive. However depending on size they might be easier to apprehend or overpower.
And some dogs of a certain temperament are likely to be calmer, and more obedient and compliant if treated well, given appropriate exercise, in the right environment and trained to be socialised.
But as pp hae said you can't out-train certain behaviours that are in-built into the breed. and the sheer size, jaw strength and muscle of them means that however conscientious someone was with training or exercising them, if they do attack they are extremely unlikely to be able to control or stop them.