Any drawback/weakness of double token stable coin compare to fiat money back stable coin like USDT?
Well, USDT is extremely easier to implemented because it's centerlized service. Otherwise, I don't think there is any drawback/weakness for this kind of project over USDT or TUSD.
I'm thinking that money-wise; fiat-back stable coin is less risky in case of crypto currency crashing. In this sense, USDT holders basically still can claim back equivalent dollars when the crypto space disappear. What do you think about this?
IOU token is less risky in case of crypto currency crashing, yes, because it basically is fiat money being recorded in a blockchain, no more nor less. The same way your money is recorded in a database of a bank.
IOU token is much more risky than Segniorage Shares stablecoin in term off trust (service security) and technical security. In Segniorage Shares,
no one owes anyone anything, so your money cannot be stolen nor cheated. While Tether can always run away with your money, or lies about a security breach to declare bankrupted.