I have seen that Tether is not backed 100% by USD dollars. They claim to backed also by loans and other digital assets. It is not a problem at all until debtors will not stop paying their debts, but it is happening, right?
Tether is a project sitting on a time bomb and when it explodes, it would be one of the biggest scam of the history like that of bitconnect, tether has been over hyped for too long and their secret is beginning to open to people and one of it is what op and you have mentioned. They claim to be stable but we still experience some fluctuations a lot in it and I think it is just one of the thing they are using to make money, just like banks.
If I am to consider the real stable coin, I would be looking at USDC alone because this stable coin is what I have seen that their value never changes since they were created, so they are the ones that truly portray the real stable coin for now ill maybe other ones like libra is being released.