USDT has the reputation couple of years ago about not having fully backed by US$ assets, which they are pegged to. This is the reason why a lot of crypto users are not totally trusting this platform. We don't know if they are now saying the ultimate truth about their assets. But on the note about investing in stable coin, the user should bear in mind that investing on this type of coin will not give him profit as it is designed to be stable in price. So your $1 for example will still be $1 after a year.