Question banks that are too small: leaked on eastern forums immediately, candidates show up reading answers out to you (some of the guides include guidance on when to pretend to think, I am not kidding).
The idealized version of "question banks" might work. The real one does not; you'd require employees constantly scouring forums in every language known to mankind, immediately removing anything that gets leaked. On top of that you'd probably require a competent committee overseeing all questions in the bank constantly and ensuring the lack of variance in difficulty.
Source: I interviewed at and for Goog and Pltr.