It is sustainable, I've seen as high as 15% APR for USDT staking but that only means that the percentage they project to you are not forever. It's going to adjust from time to time so take time and read the disclaimer that they have for this kind of product because that's the twist there. You'll get to see how these are going to change over time when the market is also changing. If I am going to take these APRs, I will only put an amount that I can afford to lose. Remember the hacks that happens? that's why it's still risky to put these USDTs or any stables into these platforms.