Terra's stablecoin wasn't so much a scam as it was built on a house of cards. They backed it with their own coin and a dynamic mint/burn algorithm, basing it all on the assumption that they'd never have that kind of crisis. In the end tho they did and it became a feedback loop.
Circle's treasury is (iirc) 1:1 with USD and T-Bills. If those fail the way Luna did then there are bigger things to worry about.
They aren't inherently a scam, but that doesn't mean that they can't be part of a scam.
Luna had a "stablecoin" but that turned out to be a scam, but that doesn't mean all stablecoins are scams.