Centralize exchange can easily manipulate Dai stable currency by listing against Fiat currency USD on it's platform.
What happens if government and centralize exchange decided to take on Dai stable currency? DAI is not immune for stability. Centralize exchange could list DAI against Fiat currency USD or USDT and manipulate it's price. If DAI value goes down against USD at any centralize exchange, investors will take advantage by buying DAI at lower price than 1 Dollar and gain by purchasing Ether from it's decentralize exchange. How could it defend against such attack?