When you hold BitUSD the value of your holdings will remain pegged to the dollar so long as BitShares itself has reasonable volatility. Reasonable volatility in this case means that it can handle greater volatility than Bitcoin has ever seen in its lifetime. The price of BitShares would have to fall to less than 1/3 its starting price in less than 24 hours and then stay there. No legitimate, widely adopted cryptocurrency has ever seen that kind of price movement. This means that BitUSD is secure against just about everything but an unfixable software bug in the BitShares protocol itself.