Even for overcollateralized ones like Dai? There's an arbitrage mechanism that forces the overall backing to be greater than 150%. Now it may perhaps be somehow conceivable that the value of the backing drops by more than a third within a single block, but that still makes it an order of magnitude more stable than banks, which usually operate with ~5% equity (and that's post 2008 regulations, and based on rather optimistic book values; the market capitalisation is often significantly less).
Even for overcollateralized ones like Dai? There's an arbitrage mechanism that forces the overall backing to be greater than 150%. Now it may perhaps be somehow conceivable that the value of the backing drops by more than a third within a single block, but that still makes it an order of magnitude more stable than banks, which usually operate with ~5% equity (and that's post 2008 regulations, and based on rather optimistic book values; the market capitalisation is often significantly less).